-58.-

CHAPTER 5
Formation of Index Languages
The Indexing described in the last chapter provided a number of different index
languages: first, one consisting of single terms in the natural language of the documents indexed; second, this initial language made more precise by the recognitionof 'concepts 1 , reflecting a first level of interfixed relations; third, a yet more precise
language recognizing a further level of interfixed relations in the form of 'themes1;
fourth a language in which the relative importance of the terms was recognized (inthe form of weights). Combinations of these provided still more precise languages;
e . g . , combination of the third and fourth.
Insofar as the indexing recognized substantive or lexical elements primarily
and lacked the relational, or syntactic device of role indicators, it was something
less than completely exhaustive. But apart from this, all the precision devices had
been accommodated and the next step was to establish facilities for expanding theelementary classes and forming further index languages - i . e . , to construct recall
devices. This chapter deals with this activity in relation to
1. Single terms
2. Simple concepts
3. A pre-established thesaurus
1. Single-term classes
A preliminary task was to prune the natural language indexing of certain minor
inconsistencies and variants which had inevitably crept in and which were not inthemselves regarded as sufficiently serious methods of defining classes to warrant
separate measurement. These initial controls involved the following:(1) Singular and plural forms were confounded;(2) American and English and other variant spellings were confounded; e.g. gage andgauge, fiber and fibre, Von Karman and Karman.
(3) Certain qualifiers of terms (affixes, hyphenated-forms which were sometimes
separated, etc.) were disregarded; e . g . , built-up, pitch-up, rolled-up, etc. weretreated as built, pitch, rolled; ellipse-like, jetlike, etc. were treated as ellipse, jet.
(4) Numbers as qualifiers were separated and treated as separate terms; e.g. Mach 6
became 'Mach1 and 'B1, N. P. L. 18 x 4 (a wind tunnel) became f N. P. L. • and '18 x 4'.
Table 5.1 gives the basic data regarding the number of single terms and their frequency
of use after the above preliminary controls had been imposed. The full set of indexing
terms is given in Appendix 5.1
Salient points are: for a collection of 1,400 documents the total vocabulary
was 3,094 terms, with reductions to 2,668 and 1,816 for the less exhaustive
vocabularies, (the reduction being based on the weights assigned to each term).
The average number of terms used to index a document was 31.3, reduced to25.2 and 12.9 respectively for the less exhaustive vocabularies. (A discussionof the problem of reduced vocabularies appears below).
As to the use of different terms, whilst the average number of times a term wasused was 14.2 this is not a very significant figure in view of the wide scatter. Of the
3,094 terms, 1,169 were used only once; one term (Plow) was used 942 times, another(Pressure) 720. The distribution curve for word-use is shown in Table 5.2 where it
is compared with three other indexes, with larger vocabularies. It can be seen thatthe distribution behaves as expected in view of the fact that it reflects a smaller
vocabulary than the other three. In fact, the frequency of use proved to be remarkably
consistent with the well-known Zipf distribution of words according to their frequency
of use in natural language texts. It will be seen that some 10% of the terms (the most

- 59 -

Collection size
Total postings of terms
Average postings per document
Total unique terms

1400 documents
43,857

31.3
3004

Variations in exhaustivity

Maximum exhaustivity (all weights)
Medium exhaustivity (Weights 7/10)
Minimum exhaustivity (Weights 9/10)

Use of terms
Average usage per term
Terms used once only
Terms used more than once
The first ten terms, ranked by usage:

Total terms
in vocabulary

Average Postings
per document3094, 2668
1816

31.3
25.2
12.9

14.2
1169
1925
Flow (942)
Pressure (720)Boundary (512)
Layer (512)Distribution (442)Theory (400)
Velocity (360)Supersonic (352)Mach (344)
Equation (312)

Variations in vocabulary size (according to different index languages).
Language 1 (Natural language, single terms only)
3094
Language 2 (Lang. 1 with synonyms confounded)
2988
2541
Language 3 (Lang 1 with word forms confounded)2444
Language 4 (Lang 1 with synonyms and word forms confounded)
1217
Language 7 (Lang 1 with minimum hierarchical reduction)
796
Language 8 (Lang 1 with medium hierarchical reduction)
306
Language 9 (Lang. 1 with maximum hierarchical reduction)(383 Proper names are not included in the counts for languages 7,8 & 9)##*'.

FIGURE 5.1 NATURAL LANGUAGE SINGLE TERM DATA

- 60 -

100
90 V-

erms

80

I-

70

H

ftslib-Crv»«P.eU fcoqifTmrm^

S

Industrial (4780 Terms)

3 60

ASTIA (7030 Terms)

MHO

AEC (12704 Terms)

& 50
«
«i
D
E*! 40

£

•a
cd
•a 30

20
10
I

0
10

20

30

40

SO

60

70

80

% of Index Terms(Ranked by Usage)

FIGURE 5.2 PATTERNS OF TERM USAGE(originally(Original figure taken from "Centralisation
and Documentation", Arthur D. Little Inc)

90

100

- 61 -

used ones) accounted for 68% of the indexing postings, and 30% accounted for 92% of
the postings, after which the curve flattens out.
Reduced vocabularies
Some explanation of the problem of vocabulary reductions referred to above
seems desirable. Generally speaking, all recall devices imply a smaller vocabulary(with bigger classes), and precision devices imply a larger vocabulary (with smallerclasses). A class is enlarged by confounding two or more classes which previously
had a separate existence; contraction is the reverse process. By 'vocabulary1, wemean the total number of discrete indexing elements, lexical and syntactic ( i . e . ,substantives and relational terms) provided in an index language. It may seem surprising that links are included in a statement of vocabulary size, since they are notdiscrete devices in the sense that they are countable in the way lexical terms androles are, but vary with the number of documents indexed. However, by the fundamental criterion of whether they define particular classes which would not be distinguished without them, they must be regarded as part of vocabulary size.
It should be noted that vocabulary size, under normal indexing conditions, is notnecessarily a determinant of the specificity possible in an index language. This isbecause increased specificity is always obtainable by coordination; e . g . , if the
vocabulary contains the terms Flow and Supersonic, class Supersonic flow is specifiableby coordinating these two terms. Theoretically it is possible to specify almost anything in this way; e . g . , Air x Cushion x Vehicle is a simple conjunction of the separate
terms normally used to name this thing; but even where a name in no way defines thenature of the thing it represents, it may be specified uniquely by contrived analytical'definition1 e . g . , in the W.R.U. Semantic Code,Tempering is represented by Process
x Metal x Heat x (number) where the number is an arbitrary code symbol distinguishing
this particular heat process on metal from any other. Perhaps the extreme exampleof the use of reduced vocabularies, with precise description resting on the various
conjunctions of a few fundamental terms was the Malvern experiment (Ref. 25).
In the case of single-term classes, without coordination, however, a reduced
vocabulary can be an absolute bar on the specificity possible. If no coordination isused, a single-term vocabulary of 1,500 specifies only half the classes specified by
a 3,000 term vocabulary. So far as testing devices is concerned, there are two different ways of effecting the expansion of classes. One is by an absolute reduction of
vocabulary whereby the reduction is obligatory for all searches; the other is by selective search programmes, whereby the effective reduction is permissive and may or
may not be utilized in a particular search. In the first case the reduction is measurable( i . e . , in terms of the number of discrete classes distinguishable) and in the other it is
not.
Obligatory reduction of vocabulary
Here, there is an absolute 'block reduction' (a block of classes being condensed
into one) in the number of classes recognized, and the indexer and searcher has no
option but to accept the confounding of more specific classes which is implied. This
was the case with reduction by synonym-control and by confounding of word forms. It
was also the case with the single-term hierarchies, although reduction by hierarchy
may be achieved permissively and was in fact done this way in the testing of 'concept'
hierarchies. This point is explained later on.

-82 -The use of quasi-synonyms* to enlarge classes is a permissive device; since
the final, expanded classes do not totally exclude the continued separate use of theterms confounded (as is the case with real synonyms) no figure showing the exact
degree of vocabulary reduction is possible. For example, the expanded class for
Bow is Bow + Bowing + Ahead + Front + Forward + Forebody; the expanded class for
Ahead is Ahead + Forward + Upstream. Clearly, the expansion of Bow does not
result in the obliteration of the separate class f Ahead1, which not only continues toexist but is in turn expandable by the addition of other quasi-synonyms. In an index
language which confounds true synonyms only, the reduction is once and for all andthe terms no longer have a separate identity.
At this stage of our thinking about the function of vocabulary size as the main
determinant of recall and precision, it seemed desirable to have as exact a measureof this parameter as possible. So the first testing of hierarchy took the form of a
fixed ( and therefore accurately measureable) reduction in vocabulary size.
Hierarchical reduction
The two methods of measuring hierarchy as a recall device ( i . e . . by obligatory,
block reductions in vocabulary size and by selective searching through different
hierarchical paths) are demonstrated below. Th« first example is one of a 'concept1
hierarchy - L e , , one not restricted to single terms and one place per term. This
is in order to show more clearly the two methods, and also to emphasize the distortion
which results from restriction to a 'one-place' hierarchy of single terms. The
latter results in the exclusion of some terms which are located in more generalcategories and the result is seen in the second part of the example. A hierarchicalnotation is attached to this example in order to make the permissive search clearer(in the schedules actually used, notation was purely ordinal).
'Concept1 hierarchy demonstrating hierarchical reduction
a
ab
aba
abaa
abab
abac
abad
abb
abba
abbb
abbo
abbd
abbe
abc
abca
abd

Experimental wind tunnel methods for investigating flow
Visualization methods
Using smoke, vapours, etc. (3)Vapour screen (1)Fog (1)
Wood smoke (l)Oil smoke (1)Using coatings, flows, etc. (3)Oil flow U)
Oil film (1)China clay (1)Phosphorescent lacquer (1)
Ink flow (1)Using spectrum (3)X-ray spectrography (1)Using Stroboscope (3)*'Quasi-synonyms' are terms which can be used synonymously in certain contexts,
but which are not true synonyms.(see page 68)

- 63 -

abe
abf
abfa
abfb
abfc
abfd
abfda
abfe
abfea
abfeb
abff
abffa
abffb

[Experimental wind tunnel method&J~ [Visualisation methods]
Using shadowgraph (3)Using photography. Photorecording (3)Drum camera (1)Photomultiplier (1)Television (1)Motion picture (2)High speed (1)Schlieren (2)Spark(1)Photomultiplier (1)Interferotnetry (2)Fringe shift (1)Interferential strioscopy (1)etc.

The full hierarchy provides twenty-nine classes.
The first reduction, replacing the terms marked (1) by a see reference to their
immediate containing head (e.g. Drum camera see Photography), leaves eleven classes,
nameiy
a
ab
aba
abb
abc
abd
abe
abf
abfd
abfe
abff

Experimental wind tunnel methods
Visualisation methods
Using smoke,vapours etc.
Using coatings, flows etc,
Using spectrum
Using stroboscope
Using shadowgraph
Using photography
Motion picture
SchlierenInterferometry

The second reduction, similarly replacing terms marked (2), which now include
those originally marked (1), by see references to their containing heads, leaves eightclasses, namely
a
ab
aba
abb
abc
abd
abe
abf

Experimental wind tunnel methods
Visualisation methods
Using smoke,vapours etc.
Using coatings, flow* etc*
Using spectrum
Using stroboscope
Using shadowgraph
Using photography

The third reduction similarly replacing terms marked (3), leaves two classes,
namelya
Experimental wind tunnel methodsabVisualisation methods
In this way figures are obtainable to show the exact effect of moving, say, from- 64 -

^the quite specific class Interferential strioscopy to Interferometry in general, thento Photographic methods in general and then to Visualization methods in general,and so on.
It will be noted that by the second deduction there is no separate class left
for Schlieren photography, whilst a distinct class is retained for Stroboscope. Yetin this subject field Schlieren photography i& a decidedly more important class than
StroboscopeThis suggests that reduction by purely hierarchical criteria may beunsatisfactory. When we reflect that the choice of terms within categories and the
choice of the categories themselves is ultimately a matter of literary warrant, it isreasonable to assume that reduction of classes hierarchically should not be a rigid
process, but should take note of the weight of literature in the different classes, sothat Schlieren photography, for example, might be retained although all other subclasses at that level were removed and incorporated in the containing class. For the
single term hierarchies, this line of reasoning led to the abandonment of 'pure1
hierarchical reduction and the incorporation of judgements as to the relative importance of particular classes, and the noting of word frequencies in determining whichclasses should be retained intact at a particular level of reduction.
.The above example stresses the primary function of hierarchy as a recall
device, whereby the index vocabulary is systematically reduced and the scope of eachremaining class is consequently widened (hence the greater recall). In practice,
however, by varying search programmes, hierarchical linkage allows movement inboth directions - to greater precision by refining class definition or to greater recall
by coarsening class definition. If specific indexing is assumed ( i . e . , each document,or document-theme, is assigned to its most specific class) a search may be madein a number of different directions through the hierarchy. For example, a searcher
commencing at Photorecording abf may find the amount of material there unexpectedly excessive and so decide to search a narrower class. This may be done, of course,
independently of hierarchy, by adding a qualifier or two ( e . g . , moving from Photorecording to Photorecording in high speed wind tunnel). But it may also be done bymoving down the hierarchy (e.g. , to Schlieren tests).
Such a decision implies that the links established by the hierarchy are permissive, not obligatory, and that the searcher selects from a comprehensive hierarchy
just those routes he considers likely to be fruitful. It is not certain whether figures
produced for a number of such searches would be useful in the sense of allowing firm
generalizations to be drawn since much will depend on the subject field, and on the
choice of pathways followed in searches (4) and (5) as discussed on the next page.
Assume that, as shown below a, aa, etc are terms in hierarchical relation:
a
aa
aaa
aab
etc.
ab
aba
abb
abc
etc.
ac
ad
ae
etc.

- 65 -

Then, if a request is made for ab, there are the following alternative basic
programmes which can be used.
(1) Term and species - i . e . ab + aba, abb .. . abz.
(2) Superordinate - i . e . ab + a (but excluding other subclasses of a; this is how'generic1 search is popularly interpreted in general library practice where £ might
represent a general treatise on the subject of which ab is a subclass).
(3) Generic - i . e . a + aa + aaa, aab . . . aaas + ab + aba, abb . .. abz + ac f aca,
acb . . . acz + az + aza, azb . . . azz (this is how 'generic 9 search is normally interpreted in machine systems and is analogous to search (1) where the content of classab is taken to include the individually specified subclasses aba, abb, abc . . . ) .
(4) Coordinate - i . e . a selection of the more likely classes coordinate with ala, e.g.
ab + aa + ad;, e. g., in a category of three-dimensional shapes a search for Spheroid
can be extended generically by searching under Body of Revolution, or by examining
all the different kinds of Body of Revolution (Sphere, Hemisphere, Ogive, Cone, etc.).
But some of the latter will be more closely connected to Spheroid than others (e. g.
Sphere) and an intermediate search, stopping well short of examining every species,
can be made. It is true, of course, that a 'closer connection9 between several subclasses implies the possibility of an intermediate step of division being inserted.
But we have to stop somewhere.
(5) Subordinate - i . e .

a selection of the more likely subclasses of ab, e. g. abb + abn.

It will be noted that programmes (1), (2) and (3) are obligatory; no freedom of
choice is given to the searcher, but (4) and (5) are permissive, the decision as tothe formation of the classes being at the discretion of each searcher.
To return to the matter of variations between single -term and concept hierarchies,
the shrinking of a concept hierarchy by restricting it to a one-place hierarchy ofsingle terms is seen by the following, which is the schedule given on pages 62-3 andreduced in this way.
V/2
V9a/10
VI1
VI2
V13
V14.16
VI7
V18
VI8
V21
V23
V24
V25
V25a
V25b

Experiment + Experimental
Visual + Visualization
Spectrography
Stroboscopic
ShadowgraphPhotography + Photorecording
Schliereri
SparkInterferoxnetry
Interferogram(Fringe) Shift
Strioscopy
Interferential
Clay
China

Of the 29 classes in the concept hierarchies only 13 appeax in the one-placehierarchy plus two (Interferogram and Interferential) which appear for the reasons- 66 -

explained on p. 70. The others are now distributed under the more general categories
as explained: i . e . , terms like Smoke, Vapour, Screen, Fog, etc. appear in othercontexts as well-and are therefore placed in more general categories. Under these
conditions, as soon as reduction of the original full vocabulary begins', it becomesvery difficult to maintain the sensible boundaries of a class like Visualization tests.
For if a question on this were now programmed to included such terms as Fog, etc.,
it is very likely that these in turn have been swallowed up in the reduction of the generalcategories and that their inclusion in the search programme can only be had at thecost of bringing in a number of other terms, such as Gloud, Snow, etc. which are quite
irrelevant to the context of Visualization tests.
Another drawback, related to the foregoing, is the loss of connection sufferedby terms treated in isolation and not in coordination. For example, a search in response to a question on 'Flow in channels1 would fail to draw in documents indexed by'Couette flow1 or 'Poiseuille flow1. Although there is a clear connection between theseat the 'concept level 1 of types of flow, at the level of single terms there is no connection between Channel (treated as a Structure affording a passage) and the personalnames Couette and Poiseuille. This situation reflects a practical difficulty in postcoordinate systems which rely on single terms - that of indicating connections (in athesaurus, say) when these connections are dependent on particular conjunctions; e.g.
this would imply a reference of the kind:
Channel: when coordinated with Flow
see also:
Couette Flow
Poiseuille Flow.
i
It is important, therefore, to remember that the performance results of the single term hierarchies reflect the use of one particular application of hierarchy as a recall
device - i . e . , its expansion of classes by fixed reduction of vocabulary size. Also,that this was a procedure determined largely by considerations of measurement rather
than regard for the normal use of hierarchy as a recall device in practical indexing.
There seems little doubt now that it is a mistake to regard hierarchy as an obligatory
recall device•. Its essential function is to act as a permissive device, allowing flexible
choice of class adjustment according to the demands of the question context in a way
which is not feasible within the artificial conditions of single-term hierarchies.
From this viewpoint, the performance figures fpr the concept hierarchies describedin the next section are a better guide to the value of generic hierarchy as an indexing
device.
Languages based on single-terms and embodying recall devices
Before describing these in detail it may be noted that a certain artificiality
inevitably accompanies the application of recall devices to single terms in isolation,
simply because, in many cases, words make little sense when stripped of accompanying qualifiers, etc. For example, the problem of synonymity in index languages frequently demands recognition of phrases, as when 'Ground effect machine' is equatedwith 'Air cushion vehicle 1 although at the single term level there is no synonymity
between the constituent terms; and a term»like 'effect' on its own is practically valueless as a retrieval handle (which is what any class, in indexing, aims to be).
Traditional, pre coordinate indexing has always begun with some degree of
coordination. Even in analytico-synthetic classifications, where 'elementary constituent
terms' are separated out as far as possible, there is no rigid adherence to the single
term as the basis of the language; for example, 'Ground effect machine' would be comfortably accommodated in a Vehicles facet. But coordination of terms is an extremely

- 67 -

potent precision device and whilst the measurement of its impact, alone and in conjunction with other devices (including all the recall devices) was of course essential, it
could not be included as a variable when measuring the impact of the other devices
on single terms. Completely free manipulation of classes is only feasible if we begin
with single terms; this is a basic assumption of post-coordinate systems. It was
clearly desirable to obtain performance figures for the impact of single devices on
single classes before attempting to measure the joint impact of several devicea - andeven a slight degree of pre-coordination would have compromised such figures.
Confounding of synonyms
This is perhaps the most obvious of all indexing devices and the one least likely
to be neglected even in the crudest of indexes. Much of this work was straightforward:e . g . , recognition of synonymity between such terms as Acoustics and Sound, Amountand Quantity, Calculation and Computation, Axisymmetric and Axisymmetrical, Vertex
and Apex, Viscid and Viscous. However, exact synonymity is relatively rare (there
might even be argument about some of the examples above). The commoner situation
is a partial synonymity, where terms are interchangeable only in particular contexts.
The evident richness of the English language, even in the literature of high-speed aerodynamics, led to quite different terms being used on different occasions (but often in
the same document) to represent the same thing; e. g., the notion of Proximity might
be conveyed by that term or by Near, Nearest, Nearly, Close, Closely, Off, Adjacent,Contact, etc. Two terms which might be used synonymously on most occasions would
occasionally diverge seriously; e. g., Interplanetary flight is equated with Interplanetary
voyage; Hypersonic flight with Hypersonic flow, Free flight with Free falling. But
Voyage, Flow and Falling cannot be regarded as synonyms.
The establishment of a synonym-list suffered one unfortunate drawback in thatit preceded the construction of classification schedules. Ideally, a synonym-list
in any given area should be extracted from a detailed classification; only by a systematic organization of all used terms according to their meanings can the ramificationsof complete and partial synonymity be exposed. For administrative reasons, however,it was desirable to proceed with the measurement of relatively straightforward devices
like synonyms, word-forms, weights, etc., whilst the preparations for the more difficult devices like hierarchical linkage went on.
The truth of the assertions just made was borne out when the classified hierarchies
were completed, in that a number of further synonyms, unrecognized in the synonym
programme, were disclosed. However, these cases were relatively few and we aresatisfied that the synonym-list on which the tests were made was reasonable on thewhole.
One difficult decision necessary in establishing the synonym list was whether
we should recognize variant word forms as synonyms. Whilst the usual view of synonymity excludes variant word forms as being examples of a grammatical rather thana semantic relationship, the practice of many subject heading lists, thesauri, etc.
which fail to recognize variant word forms at all is an implicit acceptance of the viewthat such variants are virtually synonymous. Certainly, in the process of indexingby natural language terms extracted from the documents, the fact that one word form
rather than another was selected was often almost fortuitous and this is shown, with
examples, in the section on hierarchical linkage. However, this argument was notregarded as acceptable; a thesaurus, etc. may fail to recognize variant generic levels
as well as variant word forms, and so implicitly donfound a genus and its species.

- 68 -

Should a synonym list accept this also as a type of synonymity? In view of the factthat separate measurement was being made of these other devices it was decided tointerpret synonymity as strictly as possible, but the joint use of synonyms and wordforms (for example) was also measured.
Another weakness, showing itself as a result of the single term basis, hasalready been referred to - the inability to cater for synonyms which appear only at
the 'concept1 level of phrases; e . g . , Flexural centre and Shear centre, Surface friction
drag and Skin friction, Uniform surface temperature and Constant wall temperature.
These considerations led inevitably to the recognition of 'quasi-synonyms1 as
a variant of 'pure1 synonymity.
Confounding of quasi-synonyms
In this device, those terms are confounded which on some occasions, but notall, are used synonymously; e.g. Subsonic and Subcritical; Compressor, Impeller
and Pump; Blunt, Blunted, Bluff and Rounded; Medium, Environment and Surrounding;
Region, Atmosphere and Material. A certain overlap appears here with the device ofconfounding word-forms; on many occasions, different word-forms would be used ina report indiscriminately to convey the same notion. The same overlap would appear,of course, with 'true' synonyms if 'conveying the same meaning' were the sole criterion.
But in a single-term index language, the extra element of context is lacking; although
the phrases 'Seasonal density variation1 and 'Variation of density with season' werevirtually synonymous in the reports indexed, if the single terms 'Seasonal' and 'Season'
are taken alone they cannot be regarded as synonymous.
With quasi-synonyms this restrictive rule did not apply, since acknowledgementof differences conveyed by variations in context is the basis of the device. So variant
word-forms were accepted, where applicable, as one type of quasi-synonym, e.g.
Flexural and Flexible, Flow and Flowing.
The establishment of synonyms and quasi-synonyms was done as the indexing
progressed, with the aid of glossaries, classification schedules, etc. We have alreadynoted that theoretically, the only sure way of tracing synonyms is by a close classifiction of the field, utilizing the defining functions of classification to expose synonymity
between terms used. However, the compilation of classification schedules was a much
longer task and, for clerical reasons, the list of synonyms was compiled first, in themanner indicated above. Again, for clerical reasons, the synonyms were worked out
fully only in the case of those which were demanded by the search programmes - i. e.,
the starting terms from the questions.
Confounding of word-forms
There is little to be said of this device, which was the simplest of all to establish. The expanded classes consisted of a comprehensive aggregation of all thevarious forms a given word-root could take, whether ,with prefixes, suffixes, participles or gerunds, etc. Examples are: Angle, Angled, Angular and Angularity;
Asymptote, Asymptotic and Asymptotically; Axial, Axially and Axis; Blunt, Blunted,
Blunting and Bluntness; Bound, Boundary, Bounded and Bounding. Their relations
to synonyms and quasi-synonyms have been mentioned and their place in one-place
single term hierarchies will be considered in the next section.
The terms used in searching, together with their synonyms, word endings
and quasi-synonyms, are given in Appendix 5.2.

~ 69 -

One-place, single-term hierarchies
By far the most difficult device to establish was that involving hierarchicallinkage. Two major (and connected) problems arose. Firstly, the arbitrary andsomewhat artificial restriction implicit in the need to place each term in one hierarchy only. This arose inevitably from dealing with single terms and meant that theassistance normally given to definition by context was absent. Secondly, the problem of interpreting the prolixities and ambiguities of the natural language index vocabulary in terms of this particular type of controlled vocabulary.
Problems of hierarchy
The term hierarchy as normally used in indexing can mean one of three different things:v\1) A generic hierarchy; i . e . , a system of subordinating some t e r m s to others
whereby only t e r m s which reflect the relationship of being kinds of a thing aresubordinated to Jhat thing. Other relations are excluded. But the basis for the formation of the species may or may not be a (fundamental1 characteristic.
2) A strict genus/ species hierarchy, differing from (1) in that it is confined to theuse of 'fundamental 1 characteristics; e . g . , Methane could not be subordinated toFuel (as it is in the test schedules) since a fundamental definition of Methane does
not require characterization by this attribute. A parallel has been drawn by Gardin(Bef.26) with the distinction between paradigmatic and syntagmatic relations, theformer reflecting permanent or necessary relations and the latter temporary or
contingent ones. A modern faceted classification uses both types of hierarchy in thatthe same term might appear in two or more different facets according to its status(as a product, an agent, etc.) and not be confined to the facet where it 'fundamentally'
belongs.
3) A hierarchy which includes generic and non-generic elements; i . e . , one which
subordinates some terms to others regardless of the relation involved, so long as
the subordinated term can be seen to belong to some category or facet of the 'containing' class, e . g . , the subordination under a term of its properties, parts, processes, e t c . , as well as its kinds. This situation i* typical of nearly all existing
library classifications.
Reasons why (3) should be treated as a separate device ('non-generic hierarchicallinkage') have already been given and are not considered here. In choosing between(1) and (2) for single-term hierarchies, logic seemed to suggest that (2) be chosen;
for if each term may go in one hierarchy only it is arguable that that one place shouldat least reflect the most essential characteristics of the thing represented. On theother hand, the practical purposes of hierarchy in indexing would sometimes be illserved by such an arrangement. This purpose is to provide for each term a set ofclass-mates standing in the same relation (of Thing/Kind) to the containing classand thus facilitate the expansion, or contraction, of any given class by the inclusion
or exclusion, of some or all of these helpfully related neighbouring terms; andhelpfully1 here depends on the subject context.
If the terms are relegated to a 'fundamental* or 'common' category, these helpful relations tend to become tenuous; e . g . , if the term Upper is located in a highly- 70 -

generalized category of Spatial Phenomena, its class mates will eventually include
such terms as Underwater or Buried. In a question on the Upper atmosphere, if expansion of the first term brings in such classes it is clearly unhelpful. Many otherexamples could be given; e . g . , Boosted and Reinforced could reasonably be assigned
to a basic category of activities affecting the dimensions of a thing. But in the collection indexed these terms appeared in quite different contexts - Boosted under
Rocketry and Reinforced under Structures.
The procedure finally adopted was a compromise. Where, in the test collection,
an index term had appeared only in one particular context it was placed in a hierarchy
reflecting that context, without regard to whether it was a necessary or contingent
relation. For example, Gun would be regarded as a method of propulsion in any
fundamental hierarchy; but in the test collection it appears only as a designation ofa kind of aeronautical testing device (a special kind of wind tunnel) and so it was
located with the latter. An extreme example would be Gamma; as a single term, this
could hardly appear in any •fundamental' category other than Letters; in the test collection it appeared only as the designation of a kind of steel and was located as such.
If, however, a term appeared in several different contexts suffering a significant
qualification of meaning, it was placed in a 'fundamental' category; e.g. Integral
appeared in its mathematical and structural sense and was therefore placed in a
Common properties category. Similarly, the term Working appeared in two main
guises: to designate a particular section of a wind tunnel and to designate a fluid( e . g . , a test gas). The sense of the term Working is significantly different in thetwo contexts and it was therefore relegated to a common properties category.
Problems of terminology
Closely related to the above problem was that of interpreting the intended meaning (from the point of view of the test collection) of the terms used in the natural language indexing. The organization of terms into hierarchies constitutes a form ofcontrolled vocabulary, of course; in this case, it was a control being exercised retro
spectively, after the indexing stage. The object was to place each term in the hierarchy to which it would have been assigned had the indexing been done using the controlled vocabulary. So where the same essential notion was conveyed in various
grammatical styles, these variants would have been ignored and one form done service for all; that is to say, the particular grammatical form of a term might have to
be disregarded since its semantic content in the index was the only point of interest
now. For example, a writer might refer indifferently to 'reduction of x by compression 1 , or 'reduction of x by compressing' or 'reduction of x compressively' withoutwishing to convey a significantly different idea. Again, any one of the phrases 'platewith curvature', 'curved plate', 'curve of the plate', and 'curving the plate' might
be used in a report without any intention of conveying different nuances of meaning( i . e . , without meaning to refer to the structure or the property or the operation inparticular). Other examples were Test and Testing; Calculation, Calculating antf
Calculated; Asymptote, Asymptotic and Asymptotically. All these variations in expression were ignored and the different forms juxtaposed in the hierarchies.
Where different word forms reflected significantly different emphases in meaning they were assigned to their formal categories, So Buckle and Buckling appeared
as processes and Buckled as a property; Cantilever was used to characterize a kindof beam, but Cantilevered designated a type of structure. Scooped appeared as aproperty and Scooping as a process.

jf';^^•*,,<y y:

WVfFWl^'''- 71 -In the same way, there were numerous examples of terms which appeared torepresent operations or processes (if one regarded only the single terms in isolation) but which represented an integral part of the specification of a particular kindof thing; e.g. Settling chamber, Driving gas, Non-lifting wing, Geared elevator.
Wherever such a term had appeared only in that particular context and its function
as a class determinant had been to characterize the entity and not the operation,
property, etc. , as such, it was subordinated in the hierarchy to the entity which it
specified.
The exact status of these variants on insertion into the hierarchies created aslight, theoretical problem. The confounding of synonyms in an earlier programme
had already established what t e r m s were exactly synonymous and it would have been
inconsistent now to add these variants as synonyms (the weakness of a synonym programme derived before the establishment of a classification has already been noted).
So they were simply clustered together as though coordinate in relation to each other.
Had the measurements of single-term hierarchical linkage taken the same form as
in the later 'concept hierarchies 1 , whereby various hierarchical trails were followed
in order to distinguish sharply between different relations (subordinate, superordinate,
coordinate, etc.): this might have produced a very slight distortion of the performance
figures. However^ the measurement of single-t^rm hierarchies only took the form ofblock-reductions in vocabulary size (in the manner discussed earlier in this chaper), so
no harm was done.
It must be admitted that a few e r r o r s crept in, when unjustified violence wasdone to a category by the subordination of one of its members to another category. ' •For example, in the overwhelming majority of cases, the term Revolution occurredin indexing as part of Body of Revolution1; so, according to the reasoning above, it
was located in the category of Shape, since its function was to designate a particularkind of shape. However, its synonym, Rotation, occurred once or twice in its fundamental guise of a process; it is therefore misplaced under Shape. It is not thoughtthat thesejoccasional lapses were serious. We have already seen that in making single
term hierarchies, if a term is relegated to a fundamental category this results inclasses sometimes being drawn in which are unhelpfully associated; this is also what
happens in the case of a lapse like the above.
Construction of single term hierarchies
Having settled on the various solutions to the problems described above, theformidable task of organizing the 3094 terms of the natural language proceeded. The
basic operation was one of facet analysis (a facet being a hierarchy). A useful framework for the initial sorting was the Facet Classification compiled for the first AslibCranfield Project by J. Farradane and B. C. Vickery, although high speed aerodynamics(the subject of this test collection) tended to concentrate itself in only a few of theareas covered by the scheme, and was in far greater detail than had been handled
before. Particularly large categories were those relating to Bodies, to Shapes, andvarious Spatial and general relations, %o Fluid dynamics proper, with particularclusters of detail under such topics as Compressors, Upper atmosphere studies,and Astronautics. The speed with which the last subject has developed in recent
years was reflected in the fact that whereas the Facet Classification barely mentioned
it, in this test collection it was a major theme.
Because no attempt was made to establish 'fundamental* categories as such,the common categories which were formed tended to be residual ones in that they
contained only those terms which had not found a place in a more limited context.

- 72 -

Al
A3
A^
A5
A6
A7
A8
A8a

Particles
Electron + Beta
ProtonAtomIsotope
Ion
Molecule
Mol

ASb
A8c
A8d"
A9
A9a
A10
All
A12,
A13
Al**

(Structure)
AtomicMolecular
Bimolecular
Homonuclear
NuclearMonatomic
Diatomic
Polyatomic
Polymer
Polycrystalline;

A31*
A35

Matter + Material(By use)Pigment
Lacquer
Phosphorescent
Ink
Injectant
Fuel
MethaneEthylene
Hydrocarbon
Methanol'Propellant.
Explosive'
TOT
Coolant
LubricantRefractory
Oxidizer(By origin)
Eleotrodeposit
Electroformed

A38
A39
A39a

(By constitution)
Metal
Alloy
Bimetallic

A15
Al6
A17
Ai8
A19
A19a
A20
A21
A22
A23
A2k
A26
A27
A28
A30
A31
A32
A32a

FIGURE 5.3

SAMPLE SHEET FROM SCHEDULES OFSINGLE TERMS

- 73 -

Nevertheless, some of them were still exceedingly large and detailed - e . g . , thosereflecting spatial and shape characteristics. For these, and for the common categories of Properties, Processes, Operations, etc. the Thesaurus and Code Dictionary
FROLIC produced at the David Taylor Model Basin (Ref.28) proved very useful.
Interpretation of the various word forms, etc. referred to above was assistedby the file, compiled during indexing, of synonyms, definitions, decision^, etc., by
the word frequency list, and by reference to the indexing sheets of individual documents where necessary.
Sample excerpts from the single-term hierarchies are given in Fig. 5. 3. (Thecomplete schedules appear as Appendix 5.3.) It must be emphasised that only thoseterms appear which were used in the indexing of the test collection. Whilst thisresulted in very detailed schedules in some areas, these still cannot be regarded as
exhaustive of the terms in the particular area. Sometimes, if they did not happen
to occur in the test collection, quite important terms will be missing.
2. Simple concepts
In the previous section we described the establishment of index languages
based entirely on single words, and indicated the limitations on the performance ofsynonyms and hierarchies imposed by this restriction. These limitations were accepted in order to allow the examination of the performance of the different devices
applied to single terms, in the absence of amy element of precoordination. The
next step was to accept a degree of precoordination from the outset.
Examples have already been given of the sort of simple linking necessary if
the meanings of some expressions in the natural language are not to be quite lost;e . g . , 'Ground effect machine1 must be retained as a single concept if loss of meaning
is not to be suffered. The original indexing had, of course, included a statement of
the 'concepts1 in each document - it wa6 in fact the first step taken in the actual procedure of indexing a document. These concepts were now taken as the basis for the
production of new synonym and hierarchy languages.
•Concept' languages
In order to reduce the task of preparing these to reasonable proportions it was
decided to take a substantial subset of the full collection of 1400 documents and to
make a detailed classification schedule, for all the terms appearing in it. The subset consisted of some 200 documents, containing all the documents relevant to some
40 questions. In order to make the new collection reasonably homogeneous, only
aerodynamics documents were included.
The performance of the index languages in this same subset was subsequently
measured separately for a controlled language (based on a thesaurus) and for the'options1 investigated by G. Salton and his colleagues at the Harvard Computation
Laboratory (the SMART system). Figures for the single term languages for the subset had already been obtained - they had simply to be extracted from the figures forthe.full collection.
No reindexing was attempted, of course, since this would have invalidated
comparisons with the single-term tests. One adjustment was made, however; the- 74 -

original concepts, based closely on the natural languages of the documents indexed,reflected a degree of pre coordination which was excessive for our purpose, so overelaborate phrases were now broken up into smaller units, e.g. , Biconvex circular
arc cross section was broken up into Biconvex cross section and Circular arc crosssection; Dissociated frozen hypersonic laminar boundary layer became Dissociatedboundary layer, Frozen boundary layer, Hypersonic boundary layer and Laminarboundary layer. If search were subsequently necessary for the original concept, it
would still be possible by postcoordination. Meanwhile, this splitting up allowed
maximum freedom in distinguishing facets and subfacets (arrays). In other words,
the rigidity attending the excessive precoordination typical of the older classification
systems (resulting in the obscuring of the multiple relations between facets and subfacets) was avoided.
Formation of concept hierarchies
This task proceeded in the normal way, by the now well-established process offacet analysis. However, some of the problems which occur when making a specialclassification were absent or greatly reduced; at the same time, the unusual basisof the schedules (the 'natural language1 concepts, already embodying a certain degreeof precoordination) raised some new problems of presentation. These points are
discussed later.
The procedure was as follows: the concepts (mostly short phrases like Tumbling
entry, Centre of rotation, Crossed flexure pivot, but with some single words, e . g . ,Strips, Trajectory, Pivot, Inclination) were first grouped into the following major
subject areas:
Aircraft types and parts
Bodies (Aerodynamic)
Non-aerodynamic structures
Flight: flying operations
Fluids, gases, atmosphere
Fluid flow: Kinds, Elements (vortices, jets, etc.)
Aerodynamic forces and loads, processes and properties
Aeroelasticity, flutter
Aerodynamic reference parameters (angle of attack, planform, etc.)
Mechanics, dynamics
Heat
Research: Experiment, Theory
General properties and processes.
No particular significance attached to this order; for convenience of reference it approximated to the order of terms in the original Cranfield Facet Classification.
Generally speaking it reflected the citation order used in locating concepts; a concept
containing notions from more than one area was located under the one appearing
first in the above sequence; e.g. , Wing-body interference went under Wing-body,not Interference; Spherical segment nose went under Nose (Aircraft parts) not Spherical segment (Bodies); Leading edge stall went under Leading edge. But where a clear
relation, explicit or implicit, existed between two elements of a concept, and reflected a clear precoordinate indexing principle ( e . g . , subordination of agents to theoperations or processes they serve) this was observed, even if it ran counter to thebroad rule above; e . g . , a Shielding mechanism is a structure (non-aerodynamic)

- 75 -

but it was subordinated to Ablation cooling since its functions in the literature indexed
was that of an agent of the cooling process.
The fact that Heatshield was subordinated to Ablation cooling devices did notmean, of course, that it was unavailable as a member of the class Structure if thislatter subordination had also been required. It was placed under Ablation solely on
the score that in the test collection, or the subset, this was its most probably useful hierarchy. The concept schedules were essentially 'one-place 1 schedules in linear
sequence, in the sense that no attempt was made to repeat one concept in several
different positions should it happen to belong usefully to several different hierarchies.
This last event was provided for by the rotated A/Z index, and by references within
the schedules described later. It must be emphasised that the function of these sched'ules was simply to show as cliearly and as Comprehensively as possible the hierarchicalrelations (generic and non-generic) between the terms (concepts) so that searches could
be programmed from them. The major relations were most economically displayedby physical juxtaposition. Other hierarchical relations were established via the A/Z
index and by internal references (linking, for example, Heat transfer, subordinatedto Thermodynamic processes, to Transport properties in general).
The index in the physical sense (the matrix of index descriptions and document
numbers) consisted of the separately entered concepts designed to be searched postcoordinately (the strip, or the scan-column,.method used is described in Chapter 6).
So a compound like Fully developed laminar channel flow could be sought equally in
any of the various arrays concerned, or combinations from them - Fully developed
flow, or Laminar flow, or Laminar channel flow, etc.
It follows from the above that the problem of citation order was very much
reduced, compared with a real life schedule, since it was confined entirely to the
choice between two (and sometimes, but rarely, three) elements; e.g. Jet noise,Interference rocket, Laminar boundary layer heating, Surface stress, Slot blowing.
But whilst these particular examples offered a serious choice between two or threeequally important elements, the great majority did not even demand this; they consisted of combinations such as High pressure ratio compressor, or Hinged flap,
where the major.element was obvious and the other elements trivial; no hierarchy ofRatios, or things High, or things Hinged was necessary. Since the concepts represented the limits of precoordination, the problem of providing for the accurate prediction of the exact location of all potential synthesized combinations (a major functionof citation order) did not arise. The problem of distributed relatives' was solved bypost coordination of the concepts.
It also follows from the above that problems of notation were virtually nonexistent. A purely ordinal notation to identify quickly the location of particularsimple concepts was the only requirement. Problems of hospitality and expressiveness did not arise; no additions to the schedules were contemplated and no aids todisplay were necessary in schedules which were relatively homogeneous and designed
for internal test ^programming entirely.
Within each major area the various facets and arrays (subfacets) were now
distinguished. At this point the problem of displaying generic and non-generic hierarchies arose; it was met as it usually is in conventional library classification, by
subordinating to a thing its various categories - its kinds, parts, properties, processes, etc. Below is a brief extract from the schedules which are given in full inAppendix 5.4, followed by an explanation of some of its features:

- 76 -

E64
E65
E66
E67
E68

Compressor
Centrifugal + Radial flow compressor.
+ Radial flow turbomachine
Axial flow c. + A , c , + A.f. turbomachine
Drum construction
Disk construction

E69
E70

Axial flow compressor blade
Naca 65 (12) 10 Blower blade

E71

Jumo 004

E72
E73

E89
E90
E91
E92
E93
F19
F20
F35
F37
F91
F92
F93

Single stage compressor
Multi stage compressor(etc. - i . e . , other kind? of compressor)[Parts][Stage] Q.
Stage characteristic
Sufficiency + S. performance
Cascade losses
S. matching(etc. - i . e . , other Stage characteristics)[Blade]Rotor blade
Stator blade(etc. - i . e . other kinds of Blade)(Blade characteristics)
Blade shapeB. curvature(etc.)[Flow phenomena]Ir^otational flow
[Rotational flow
Inlet whirl
P r e whirl(etc.)

The first subclasses under Compressor are Kinds of compressor: Centrifugal andAxial flow (based on direction of flow): Single stage and Multi-stage (based on stage
numbers) and so on. The synonyms which appear at the concept level automaticallysort themselves out (e. g., the three variants at E65). Any categories (generic andnon-generic) which refer to a given subclass follow that subclass immediately. So
under Axial compressors is found Kinds (Drum construction, Disk construction, Jumo004) and Partis (Bllade) - and a particular kind of axial compressor blade follows that.
(N.b. - a clerical e r r o r has resulted ixi the Kind of a.f, c. 'Jumo 004' following thePart, 'Blade 1 instead of preceding it; such e r r o r s did not affect the programming ofsearches).
The Kinds facet is followed by the Parts facet (Stage, Blade, e t c . ) . The bracketed
term Stage followed by »Qt indicates a term which appeared in one of the questions but
not in the indexing of the subset documents; it has been inserted for programming purposes. The a r r a y of 'Stage characteristic* demonstrates a recurrent problem in thesubject field analysed, that of maintaining 'generic 1 relations in a situation wherestrict definition of terms would result in an uncomfortably large number of tiny subfacets

- 77 -

consisting often of a sole member; e . g . , Stage efficiency is evidently a property andStage performance a process - but they are treated as virtually synonymous; Cascadelosses constitute a factor in efficiency or performance, but hardly a 'kind of efficiency1.
Stage matching is another concept which lies on the borderline between processes
and properties. It is possible to say, however, that all these rather subtly related
notions are 'Stage characteristicfirand in this way the facet structure is maintained
without undue complexity. Other examples of a certain amount of violence being
done to the strict netture of generic relations may be found, as at M4/33 (see Appendix
5.3) where complexly related terms are grouped as Atmosphere properties andcharacteristics, or at P7/26 Processes and properties of Vortices. Similar situations inevitably arose in the single-term hierarchies in areas like Mechanics andDynamics where the conditions of what Ranganathan has called 'canonical1 classification tend to hold.
Another minor liberty, not demonstrated in the example above, was taken in
the treatment of qualifying terms like Theory, Approximation, Experimental data,when these were found precoordinated as in Hypersonic flow approximation. It could
be argued that these do not narrow the extension of the term they qualify and should
therefore be disregarded - i . e . , treated as synonymous with the term alone. Theoretically, this is why they are usually placed (in the guise of 'Form divisions') at
the very beginning of the subdivisions of a term in conventional classification. In
the search programmes, however, they were included in the 'Terms and species'
sub-programme. This was later seen to be a mistake, but it is not thought that thiswas serious in view of the very few terms involved.
Although the differently related facets follow and interrupt each other without
clear signs of demarcation in the schedules, the different relations were strictly observed, of course, when the search programmes were compiled; i . e . , when expandinga class by generic hierarchy, only those terms standing in a true generic relation tothat class were counted; e . g . , Compressor + Centrifugal c. + Axial flow c. + Drum
construction + Qisk construction + Jumo 004 + Single stage c. + .. . would be given
as the full generic expansion of a particular kind of compressor (see the Generic Broad search below). Any terms not standing in a true generic relations ( e . g . ,Axial flow c. blade, Stage characteristic*, Irrotational flow, etc.) would be ignored.
Multiple hierarchical relations
The major weakness of the linear display of classes just described, in whicha particular class (concept) is located in one place only (albeit a carefully chosenone) is that it fails to show the further generic relations a class may have. For
example, Jet interference is subordinated to a category Jet characteristics, in which
its class mates are Jet exit, Jet location, Jet energy, Jet structure, Jet emission,etc. It could equally well be subordinated to a category of Causes of interferencewith class mates like Wake interference, Forebody interference, Support systeminterference, Wave reflection interference, Wing-Body interference, etc. But inthe schedule described these last terms are 'distributed relatives'.
There are two traditional methods of handling this problem in a real-life classified index: by multiple-entry as with UDC, where the number of entries for a given
compound-class-description are multiplied, and filed according to a different classification, so that Jet interference appears in a class Jet (divided into Jet .characteristics)
and also in a class Interference (divided into Causes of interference). Or, by leaving
these other connections to be indicated by an A/Z relative index, in which all ther 78 -

different contexts in which a term appears are gathered together as qualifiers of that
term.
Although in a real-life situation the first method provides easier access to thesefurther relations, this advantage was not significant in the test environment. The
test collection subset was relatively small and a fully rotated A/Z index was easily
producable by clerical labour, In any case, such an index was necessary for otherreasons, too, as will be described. Moreover, although the working out of full alternative hierarchies would have involved a considerable effort, there was no guaranteethat more than a small fraction of them would ever be used, since only those hierarchies
relevant to the terms occurring in questions would be required.
The assumption above is that such an A/Z index will automatically disclose theexistence of other possible hierarchies. Indeed, it is difficult to see how sucH additional hierarchies could be economically developed unless we are guided by theliterary warrant afforded by the actual occurrence of the terms concerned in conjunction with these other contexts, in which case the A/Z index automatically picks
them up. Nevertheless, further connections were indicated by references in theschedules wherever it seemed desirable, particularly where it seemed that the A/Z
rotation of terms might still fail to show the connection; e. g. Small disturbance theory,
subordinated to Disturbance, contains a reference to Boundary layer theory to which
it is also relevant. Streamlines, subordinated to Flow elements, has a reference toRelative stream surfaces (in Compressor flow phenomena) to which it is also generic.
Or, within a given class, references were added to link concepts occurring in different arrays; e . g . , Performance discontinuities in the Performance facet of Compressors contains a reference to Stall and Surge in the Flow facet of the same class.
It may be noted that in a real-life classified index, the A/Z index usually shows
even those connections just exemplified, since its entries contain more qualifying
material (providing further information regarding the context) than the test index,
where the concepts gave the sole element of precoordination. For example, a document dealing with Small disturbance theory in the context of Boundary layer theory
would produce rotated A/Z index entries:
Small disturbance theory: Boundary layer theory
Boundary layer theory: Small disturbance theory
and these establish the connection which in the test collection had to be establishedby references.
The significance of multiple hierarchical linkage as an element in the recall
performance of generic hierarchical linkage generally is probably not very great.
Most questions impose a particular context of their own and the likelihood of relevantmaterial being found in radically different contexts of the particular terms of thequestion is probably small. For example, a question on the kink in the surge-line ofa multi-stage axial compressor imposes a context on the notion of 'surge1; clearly,
documents indexed under Surge as a general concept should be examined, but it isunlikely that extended examination of the classes flanking Surge in the general hierarchyof Aerodynamic processes would be very fruitful.
The A/Z Index (see Appendix 5.5)
In order to provide for multiple generic hierarchical linkage as discussed above,and for other reasons, a rotated A/Z index of the concepts was now produced; e.g. ,•-79 -

Class
No.
Afterbody, Conical Base
.Cylinder.Cylindrical
Drag
Surface.Truncated
Vehicle. Conical(etc.)
Base Afterbody. Conical
Bleed
.Flat
Forward attitude(etc.)
Cone
Cone,Blunt Nose
, Blunted. Circular
CylinderCylinder Bodies(etc.)
Conical Afterbody Vehicle
Base Afterbody
Camber
(etc)

D6
D5

T16
K61
H55

H76
H76
A57
C99
W33

It can be seen that each concept appeared as many times as it had distinct words. So
the first concept above appeared in three different contexts - that of Afterbody, ofBase and of Conical. The class number appeared after the direct form of the concept.
The index served the obvious purpose of a key to location besides its other major
purpose - that of indicating all the different contexts in which a given term had appearedin the schedules. One aspect of this second function, the capacity to reveal othergeneric hierarchical relationships, was discussed above. But this was only one kindof context revealed. In the example above, Afterbody surface, Base bleed, Base
forward attitude, etc. reflect non-generic relations. The index therefore acted asa valuable supplement to the schedules proper in displaying these relations. The
major display of these was. of course, by the subordination of a thing's categories
to that thing. But these would not necessarily exhaust the non-generic relations, and
the A/Z index, not merely supplied further relations, but could lead the question programmer back into the systematic order to explore further categories, if necessary.
For example, examining the entries adjacent to Heat transfer leads to Heat sustaining
leading edge (subordinate to Leading edge), to Heat transfer at the wall (subordinatedto Surfaces and Walls, where related concepts such as Constant wall temperature andv Wall temperature gradient are found) and to Heated air (subordinated to Air. whererelated concepts such as High temperature air and Dissociated air are found). Manyof these other concepts do not contain the term 'Heat1 or its variants and might nothave been picked up had purely alphabetical considerations governed the search.
A third major function served by the rotated A/Z index was to provide a recall
device based on the •accidental1 alphabetical juxtaposition of concepts enjoying a limited

- 80 -

degree of pre coordination. It has already been shown how the cluster of concepts
around a given term (which might also be a root term for a number of word forms)
such as Heat or Interference reflects a variety of relations; e.g.,Interference, Blockage;Interference, Forebody; Interference, Jet; these all reflect kinds of interference
according to source. Interference filters reflects Interference as an experimental
agent (in temperature measurement): Interference load reflects Interference as asource of another phenomenon. When these assorted relations are added to a certain
degree of word-form confounding, (e.g. expanding an initial enquiry for Dissociationby the addition of classes like Dissociated stream or Dissociating fraction) the result
is an eclectic recall device which utilizes elements of hierarchy, non-generic hierarchy,
confounding of word-forms, and linking (an element of precoordination is essential tothe programme). Such a mixture cannot, however, rank.as a 'device1 in the way thisnotion was understood in Chapter 4. It is'further considered in the next section.
Formation of Classes by Search Programmes
A significant feature of hierarchical linkage as an indexing device is the richvariety of relations it displays, enabling a number of different paths to be pursued inadjusting the size and content of the class or classes with which a search begins.
Some of these paths were briefly mentioned in the last section, using the example ofVisualization tests.
In exploiting these relations two different policies can be followed; either classes
are expanded by bringing in all the terms related in a particular way - e.g. , all theterms subordinate to the original one, as when all the different kinds of compressors
are added to a search for Compressors. Or, classes are expanded eclectically,choosing just those members of a given relationship which seem most likely to be
relevant in the context of the whole question. The latter policy is the one normallyfollowed in the conventional classified index.
The former policy has the merit of simplicity in programming (once the schedules
are established) and this is clearly pertinent in the case of machine searching and is,in fact, generally implied by the term 'generic search 1 . Equally obvious is the factthat it will tend to result in a lower precision ratio than a selective search, but possibly also a higher recall ratio.
In th6 testing of the concept hierarchies it was decided to attempt both approaches
and the following different searches were programmed, each one producing a differently
defined class.
(1) The simple natural language concept alone(2) Confounding of synonyms. It has already been pointed out that a classification
should automatically throw up synonyms as a result of its analysis; also, that a number of synonyms only become apparent at the level of concepts. Both these factors
operated to produce a programme for synonyms quite different from that using single
terms alone. Examples are: Temperature distribution + Temperature profiles + Temperature history; Angle of incidence + Angle of attack + Arbitrary angle of attack +Incidence; Initial expansion region + Prandtl-Meyer region.
(3/8) From this point onwards, the classes formed by (2) were regarded as the basic
classes to be expanded. This expansion was achieved by adding further classes to (2)on the basis of the following programmes:

- 81 -

\ (3) Term and species: If the basic class were Non circular cylinder and its synonyms'/. (H75), this would be expanded by the addition of Cone cylinder + C.c. bodies + Elliptic c. + Elliptic c. of eccentricity \fS , + Hemispherical c. + C. with h. nose + Ogivec. model + Flat faced c. + C. without corners.
(4) Term and species (selection): a choice was made from (3) based on the context
of the question asked. For example, in a question on the kinetic theory of gases,
when programming the term Gases, only those kinds of gases which reflected insome way the problem of the question were selected .- such as Ideal gas, Real gas,High temperature gas, Dissociating gas, Equilibrium gas.
(5) Superordinate - i. e . , adding to the basic class its immediate containing genus
and as many more genera beyond that as appeared sensible; the number of steps
included would rarely exceed three. To Non-circular cylinder (H75) would be added,
for example, Cylinder 4 Body of revolution + 3-dimensional body. It should be notedthat only the superordinate, term was taken - not its species as well; the search isthe equivalent of the traditional library search under 'more general1 heads.
(6) Generic (narrow). - i . e . adding to the basic class its immediate containing class(genus) and all the other species in the same array (subfacet) as the basic class;e.g. , to Non-circular cylinder would be added Cylinder and the rest of the arraybased on circularity of shape, but excluding those kinds of cylinder (Inconel cylinder,
Flat faced cylinder, Long cylinder * .etc*) reflecting other principles of division(Material, Edge properties, Length* etc.). Similarly, if the basic class were Supersonic flow, this programme would add to it all other kinds of flow designated by speed,
but excluding kinds of flow based on other principles, such as viscosity, compressibility, degree of turbulence, etc.
(7) Coordinate (selection): a choice was made from (6) of the most likely terms, but
excluding the superordinate term. Since by definition the classes of an array aremutually exclusive this Was never a very promising search and in fact was not often
productive of any terms. But in those border line situations referred to above,
where the concept of generic hierarchy can only be realized practically by accepting
a less-than"-precise category such as •characteristics 1 or •phenomena1, the likelihood
was greater; e . g . , in a question on Air dtfag the coordinate class Atmospheric rotation was accepted. Another example is that of opposites, or near-opposites, like
Laminar flow and Turbulent flow, where a dociument frequently refers to the one evenwhen its primary subject is the other.
(8) Generic (broad): this added to*(6) as many more superordinate terms as seemed
reasonable, together with all their species - i . e . , not just those restricted to theimmediate array (subfacet) in which the basic term appeared. For example, if thelatter were Supersonic flow, this search would now bring in documents indexed byany kind of flow - Laminar and Turbulent, Conical and Parabolic, Equilibrium andNon-equilibrium, etc. This somewhatundis criminating acceptance of the complete
contents of a hierarchy is the equivalent of the 'generic search 1 as usually understoodin machine searching.
(9) Systematic Collateral (selection): this was a selection from (8) analogous to theselection from (6) which produced coordinate classes (7) - again excluding the superordinate terms themselves. This search was more productive than (7) since there isoften a close connection between concepts from different arrays of the same genus.
This fact underlies the correlation of properties and the principle of definition by- 82 -

aggregation of attributes, where a term is defined by a number of attributes, each ofwhich reflects a different principle of division of the genus which lies at the heart ofthe definition, e . g . , Poiseuille flow may be defined as Compressible> viscous, laminarflow between closely parallel planes - and each attribute reflects a different characteristic of division for the genus Flow. So where the basic class was Boundary layer
flow, for example, related classes brought in by this programme would include Shearflow, Separated flow, Viscous flow, etc.
(10) A/Z collateral: the rotated A/Z index of concepts has already been described..
This search was made first by examining the index to find the basic class (question
concept) and any other concepts containing it ( i . e . , consisting of the basic class withfurther qualifications). Those which seemed likely to be relevant were now added tothe basic class. For example, to the basic class Axial compressor was added Axial
flow compressor blade since this included the basic class and seemed relevant. Or,
to Heat transfer would be added such concepts as Convective heat transfer rate, Surface subjected to heat transfer, Laminar heat transfer distribution, etc. It may be
noted that most of these further classes represent non-generic hierarchical relationsof the basic class. Also, that most of the question concepts already consisted of two
or more words and that in many cases there would not be any more concepts containing the one sought; e . g . , this was the case with Multistage compressor, Non-circularcylinder, Dissociated stream.
For those concepts containing more than one word a »second -level 1 search was
also made, in which each significant word (and any of its adjacent variant word forms)
was examined separately and further classes selected from the total body of concepts
containing it. For exaimple, to Axial compressor woulcj be added Axial inlet impeller;
to Surge line would be added Stall limit line and Surge. It should be stressed that theseselections were made in the context of a given complete question and might vary somewhat for the same concept if the context differed. For example, in a question on the
Surge line of an axial Compressor, the 'second-level 1 for Axial compressor would
reject Compressor surge (although it wpuld be relevant to the question as a whole) because
this approach was already covered by the programme for Surge line. Again, it may
be noted that many of these further classes represented non-generic hierarchicalrelations; in addition, the combined first and second level searches generally included
those terms selected from generic hierarchies in searches (4), (7) and (9) which also
included the actual terms used in the basic concept.
(11) Residual hierarchical linkage. The A/Z collateral searches, although providing
a large number of non-generic and generic hierarchical linkages, were restricted tothose which included one or more of the terms actually used in the question. This
still left a number of possibly helpful classes excluded. They could be divided into
two groups: firstly all those from the non-generic hierarchies which appeared in theschedules where the question concept (the basic class) was located, but which failed
to include the actual question term or terms (in which case the A/Z collateral would
have disclosed them.) It was a simple matter to establish these, by scanning thevarious facets subordinated to a given concept, or adjacent to it.
Secondly, all related classes not already disclosed by the hierarchical relationsof the ten searches described. A number of these were already provided for in theschedules, by references; e . g . , Surface combustion (D66) see also Ablation; Vanelessdiffuser (in compressors) (F84) see also Ducts; Compressor surge see also Rotatingstall; Mass flow fluctuation (U44) see also Sound waves.

- 83 -

It has already been noted that the formidable task of adding to the 'one-place1
schedules all other possibly useful hierarchies WM, not attempted. This was partly
because much of the effort would have been wasted (if no questions were asked involving these alternative hierarchies), and partly because the A/Z index was likely
to disclose the most important ones. It was also thought that the detailed analysisof reasons for failure (an integral part of the test programme) would disclose any
examples of failure due to the absence of such alternative hierarchies.
It should be remembered that the hierarchies actually established were thosereflecting the most likely approaches to the material and that for many of the concepts
alternative approaches (manifested in different citation orders) were quite obviously
unnecessary; to take some at random for example, Mixture of cold gases could notconceivably enter into a search for kinds of mixtures f or kinds of cold things; thesame applies to a number of other concepts involving the term Mixture. Similarly,
in the case of a number of concepts involving the word 'modes', or 'models', it was
unnecessary to contemplate the possibility of having hierarchies based on these(although hierarchies of particular kinds of model, e . g . , wind tunnel models, wereof potential value, of course).
The references already provided in the schedules and by a file of 'notes anddecisions' assembled during the indexing were now supplemented by those in various
thesauri and subject heading lists in the field of aeronautics and astronautics, sincethese are in principle the product of similar observation of connections between
classes. Examples of such references are those from Heat transfer to Transportcoefficients, Large Peclet numbet* and Prandtl number; from Dissociated stream to
Ionized boundary layer; from Kinetic theory to Diffusion and to Transport properties.
It has already been noted that all such connections could, if necessary, be incorporated
in a hierarchy of the kind being tested, although no distinction was drawn between
generic and non-generic relations when utilizing these references.
The combination of search programmes (10) and (11) represents, by and large,
the performance of non-generic hierarchical relations largely, combined with asmaller element consisting of those ^supplementary generic relations not shown directly in the 'one-place' schedules. Although it has already been argued that both thesehierarchical relations are generally quite secondary to the main display of genericrelations, it must be regarded as a weakness of this joint presentation that separate
programmes were not made for the two distinct situations.
3. Control by pre-established thesaurus
A major objective in producing the concept hierarchies described in the lastsection was to afford a degree of precpordination sufficient to remove the artificialities accompanying the use of single words only in the 'one-place' index language
and to provide where suitable, that minimum of syntactical linkage necessary to
the clear conveyance of unambiguous meaning in the index descriptions. It wasthought that the resulting index language approximated more closely to the usualenvironment of index devices than did the first language.
By this time, the search methods developed in the course of testing the first
languages were producing the first detailed performance figures for the various
devices and languages concerned. Although the operation of the large number ofvariables produced an extremely complicated picture in that many ways of aggregatingthese variables and their different values presented themselves, the general picture
seemed to suggest clearly enough that the performances were not very encouraging.

- 84 -High recall was obtained only at a low level of precision, and as soon as the latter
was improved, a precipitate drop in recall ensued.
A number of contributory causes of this were suspected, 'fhe ambiguities andinconsistencies of the language of aertidynamics suggested one. The match between
the terms of the questions and the relevant documents which was, in some cases atleast, very poor was another. The possibility of defective indexing was not thought
to be very serious in the sense that exhaustive selection of keywords and phrases
and the organization of these into concept and themes appeared to be reasonable.
But a failure to recognize fully the connectivity between the terms of the languages
so far established undoubtedly caused some of the failures.
Another possible factor was the unusual route by which the initial concept indexing had been translated into the different languages. In a real-life situation,
this translation is done concurrently with the indexing itself, which is channelled
into the controlled language as the first stage. The central elements in the test
languages had so far been applied almost entirely retrospectively. Although there
appeared to be no reason why this should have affected index performance, it seemedthat validation of it as a method (by comparing it with a normally produced index)
would be useful.
One way in which improvements in performance were thought to be possible
was by putting more sophistication into the search programmes (by distinguishing
between terms of different potency, between different combinations of these, andso on.) It was thought that maximum discrimination and control in searching implied
the need for maximum discrimination and control in the indexing if optimum performances were to result. Again, although it was probable that the controls effected
retrospectively were as valid as those imposed concurrently (as in indexing by arecognized, pre-established, control language) the slight element of doubt suggested
that it would be wise to demonstrate this.
These considerations led to a decision to set up a conventional index with adifferent set of connectives based on a predetermined list of terms and to compare
its operation with that of the natural language with retrospective controls alreadytested. For this, the Engineers 1 Joint Council Thesaurus of .engineering terms,(Ref.28) was chosen as providing an up-to-date control language in the field ofphysical science and engineering, which contained clearly defined connectives grouped
in a manner allowing convenient comparison with a number of the hierarchicalsearches described in the last section. A second subset of 350 documents was selected; this included the 200 documents from the first subset that was used in testing
the concept hierarchies, thus allowing direct comparison with all previous programmes.
As in the case of the simple concept languages, no reindexing was contemplated,
only another translation of the indexing done originally, since reindexing would haveintroduced an immeasurable variable; but the production of the new indexing language
simulated the normal indexing situation. In this, each document is subjected first
to 'concept-analysis• when it is decided what the document is about, what its significant
terms are and how these are related in concepts and themes. This is followed by
the translation of this information into a particular index language, with pre-establishedcontrols as to the level of specificity to be allowed and the recognition of synonyms
and of other connectives between terms and between concepts.
Production of controlled index language using E. J. C.
The main problem raised by the use of E.J. C. was due to the fact that a

- 85 -

relatively general thesaurus was being applied to a special field. Although some lossof specificity (compared with the natural language) was regarded as inevitable, aconsiderable extension of the vocabulary was necessary if the specificity were notto suffer seriously. This extension raised problems of maintaining consistency withthe principles with which the existing vocabulary and its syndetic structure of connectives had been developed. To assist this, the Rules for preparing and updating
Engineering Thesauri (4th draft) November 1964 were observed as far as possible.
Selection of terms
Generally speaking, the aim was to incorporate the extra detail as unobtrusively
as possible, without disturbing the distinctive character of the E. J.C. index language.
The various E. J.C. methods for keeping down the size of the vocabulary were observed
where feasible:(i) Outright rejection of highly specific terms (Rule T-l) when the sense of the term
could be approximated with reasonable adequacy by a broader term. E. J.C. omitted
a number of prominent aeronautical and aerospace terms which did not appear tomeet this criterion ( e . g . , Sonic boom, Tail, Stall, Bodies, Buffeting, Chord) andthese were simply added. It also omitted a very large number of more precise terms
and phrases occurring in the natural language indexing but which qualified for consideration under this rule. Particularly affected were those terms reflecting spatial,
dimensional and temporal characteristics many of which were in adjectival form (which
E.J.C. avoids); e . g . , Normal, Perpendicular, Vertical, Horizontal, Behind, Outside,
Below, Nearly, Large, High, Circular, Rectangular, Octagonal, Radial, Circumferential, Zero, Rate, Without, Free.
In some of these cases, where the notion was obviously dispensable becauseof its poorness as a retrievable handle; the term was omitted. Examples of thiswere Behind, Complete, Continuous, Degree, Direct, Coefficients, Effects, Horizontal, Vertical, Near, Nearly, Normal, Outer, Outside (although Some of theseappeared in phrases, such asContinupus loading). Outright omission was usedcautiously since it diminishes the exhaustivity of the indexing. It may be noted thatthe main reason for holding exhaustivity constant is its effect on recall. However,
the absence of a term which is completely 'non-potent1 as a retrieval handle will
not affect recall except in one circumstance - the use of single term searching.
Theoretically, if a question includes the term Degree or Normal and this single term
is searched it might retrieve a relevant document which would otherwise not be
retrieved. This possibility is removed if the term is totally obliterated from theindex vocabulary. However, this situation was regarded as sufficiently remote fromreality to allow it to be ignored.
Strictly speaking, the only condition under which exhaustivity is affected byindex language (as distinct from the personal decision of the indexer to include or
not to include a notion) is when the language completely fails to provide an appropriate
term even at the highest level of generality. This sometimes occurred with E.J.C.
and the solution was simply to use the name of the category to which a term belonged;
e.g. , the term Shape was used for a whole cluster of natural language terms - Biconvex, Concave, Circular, Configuration, Diamond, Elliptical, Octagonal, Rectangular,
etc. Or, the category term Position (location) was used for terms like Beneath,
Outboard, Between. In this way,' although specificity suffered, there was no lessening in exhaustivity.

- 86 -

(ii) Confounding of opposite^: this was used occasionally, as in Continuum flow,
Use F r e e molecule flow.
. ,j(iii) Avoidance of precoordination: consistency here was not assisted by the E. J . C .
rules, one of which ( T - l ) warns against being too specific and another (T-4) warns
against not being specific enough (in the matter of bound t e r n ^ ) . One arbitrary
limitation on the degree of precoordination (to 34 characters) i s evidently imposedby the three-column format used in printing the-Thesaurus. But only in a few c a s e s
did new combinations exceed two words ( e . g . , Mass transfer cooling, Blunt leadingedge, Wing-Body-Tail configurations).
This policy included the representation of some concept? by an instructed c o ordination of single terms; e . g . , Aerodynamic noise Use Aerodynamics x Noise(sound), Dynamic s y s t e m s Use Dynamic characteristics x Systems, Sounding probes
Use Sounding rockets x Space probes, Radiating body Use Radiation x Aerodynamic
configuration, Reflected wave peak overpressure Use Shock wave x Reflection x
P r e s s u r e . This device i s not very clearly described in E. J . C . (using theft and &devices) and some of the examples of precoordination make the policy no clearer;e . g . , under the term. P r e s s u r e i s given a large number of precoordinated phrases( P r e s s u r e distribution, P r e s s u r e measurement, P r e s s u r e gradient, e t c . ) . Whena 'new 1 term P r e s s u r e plotting occurred, it was not clear whether to precoordinate
or keep separate or confound as a near-synonym of P r e s s u r e measurement. Again,
a 'new1 term Circular wind tunnel might lead to acceptance by analogy with Circulars a w s , etc. But should Rectangular wind tunnel and Octagonal wind tunnel be s i m i larly distinguished? Sometimes, this sort of economy in precoordination avoiding
highly specific new t e r m s , led to strange equivalents such as Root section. UseFoundations x Profile.
The record of these rejected terms and phrases, together with the ones to be
used in their place, grew to large dimensions and constructed a m a s s i v e 'lead-in 1
vocabulary from the t e r m s and expressions of the natural language to those of thecontrolled E, J . C . languages. Over 1,500 entries were made for the subset, which
totalled 350 documents. It should be noted, however, that a number of these rejects
were simply word-form variants, e . g . , Oscillatory Use Oscillations; Oscillating
Use Oscillations; Oscillatory motion Use Oscillations; Elastic Use Elasticity; Edged
Use Edges.
Selection of references(1) UF (Use for) These have already been considered above as forming a lead-in
vocabulary.
(2) BT and NT (Broader terms and Narrower terms) The definition of these tworeciprocal relations is reasonably clear in E. J . C .
The BT reflects a true generic (Thing/Kind) relation excluding not only the obviously
non-generic ones, like operations, or Properties, but also, explicitly, the Whole/Part relation, which is often loosely associated with the generic. The BT also e x cludes "generic families constructed on the basis of usage 11 , so Platinum, a memberof the c l a s s Metal, i s not regarded as a member of the c l a s s Catalysts since it is
only sometimes used as a catalyst, This s e e m s to suggest an even stricter interpretation by the E. J . C . of the notion of 'class 1 - i. e. , one which excludes frommembership all but 'true' species in the sense that they p o s s e s s permanent and funda
mental characteristics,* uniquely defining them.

-87 -

However, this is not borne out by an examination of the Thesaurus, which suffers some inconsistency on this point. For example, Wind tunnel nozzles gives,
quite correctly, Nozzles as afiT. But the previous term Wind tunnel models fails
to give Models as a BT and under Models gives Wind tunnel models as a Related
term (RT). i . e . , a non-generic relation. Again, the term Materials has eighteen
RTs; some of them are kinds of material based on Structure (Composite, Granular,etc.) some based on Properties or Behaviour (Radioactive, Magnetic, etc.) somebased on Use (Structural, Molding, etc,). It would appear that the first characteristic, at least, designates true species. Again, under Plastics are listed numerous resins as NT6. The term Resins itself is treated as a synonym of Polymers,
however; but no reference of any kind connects Plastic to Polymers or vice-versa,
although there is limited duplication between the NTs for each of these terms.
Similarly, there is no connection established between Pumps and Compressors (thelatter being treated as a synonym for Air compressor) - although Pumps has numerous NTs in the form of pumps of particular application, e. g. Fuel pump.
(3) RT (Related terms) These are designed to.show non-generic relations (asdefined above) and the ruled state that it is undesirable to make RTs to 'more
specific1 terms. However, there are numerous examples in E.J. C. of RTs which
do not observe this. For example, Hydraulic equipment has numerous references
to particular types of Hydraulic equipment (Hydraulic brakes, Hydraulic presses,etc.) Apart from the fact that there were cases of true species (e.g. of Hydraulicequipment) being included in the RT framework, problems arose regarding thereferences from terms like Quartz (as a heat shield); if we assumed that Heat shield
is in the relation of RT to Quartz, should we, under Heat shield, have added Quartz
as an RT? By analogy with Insulation (say), which gives as RT the material Magnesium Oxide, Quartz should have been added. But if it represents (as it does)
Quartz as designating a kind of heat shield according to material, it is a 'more
specific 1 term and such references are not encouraged.
Generally speaking, E. J. C. observes the old rule of Subject Heading lists
which avoids references to adjacent headings on the score that their juxtaposition
makes further reminders to the searchers (the question programmers) unnecessary.
For example, there is no reference between Shock waves and Shock tubes. So thisshould lead to the avoidance of a reference from Ionization to Ionosphere (which was,nevertheless, made). Also under Molding materials there are references to four
RTs which are adjacent entries beginning with 'Molding . . . ' .
Apart from these efforts to observe consistency and method in making references, the usual variety of relations appeared to be permissible and consequently
RTs were added for new terms and for existing E. J. C. terms when these had inadequate connectives. Examples of the latter situation were fairly common since
Aerodynamics is not a particularly favoured subject in E.J. C. For example, there
are no connections between Vibrations and Elasticity, or between Supersonic flow
and Shock waves; in the case of Poiseuille flow, (which may be defined as viscous,
laminar flow in pipes or between closely parallel planes), the term is rejected andreferred to Laminar flow but without any references linking it to the notion of pipe
flow, which in this context is just as important as laminar flow.
References from post coordinated terms.
E. J. C. provides a number of instances in which a term is distributed between
two or more wider terms; e . g . , Pressure gas welding-tt had BTs which include

- 88 -

'Gas welding &• and 'Pressure welding &' (the ampersands mean that the two broader
terms Gas welding and Pressure welding can be jointly substituted for the single andmore precise term Pressure gas welding). This procedure led to a number of similar new references; e . g . , Prandtl-Meyer flow Use Supersonic flow x Expansion.
However, this device leads to a difficulty, inherent in post coordinate indexing,
when these particular coordinations generate new reference structures of their own
which are not apparent at the level of single isolated terms. E . J . C . offers no guidanceon this point. For example, assume that for Conical flow the instruction is to use
Cones x Flow. In this case, some sort of reference seems desirable, either fromthe rejected phrase Conical flow or from its constituent terms in the form:
Cones: when coordinated with Flow, RT Shock waves.
In this particular case an intermediate connective was established by using a heading
Mach cones. Examples of where the reference structure could be fairly elaborate
are Flow x Parameters and Vapour x Screens x Procedures. Each of these subjects
has its own set of related terms, generated entirely by the conjunction of their constituent terms, e.g. Vapour screen method, which now has its own related terms,
such as Carbon tetrachloride vapour, Humidity control, Temperature control, Operational fog density.
The implied need for such connective references if the syndetic structure were
to be developed raised problems of complexity in the scan-column search techniques(which were designed to be purely clerical in operation). In view of this, together
with the fact that E. J.C. quite ignored such post coordinate reference needs, it was
decided to follow the E. J.C. policy and rely on future analysis of searches to show
where this weakness contributed to failures in performance.
E . J . C . roles
The E . J . C . system of roles which appear, without explanation, in Table 3 ofRef.2 8was used in the indexing, The reasons why roles were not tested in earlier
languages have already been discussed, but the availability of a ready-made set ofroles seemed a useful opportunity to investigate whether some of the assumptions
made there (those relating to the applicability of roles to aerodynamic literature)
were justified. Unfortunately, at this stage of the project, the time factor was beginning to limit the amount of new testing which could be undertaken and it was decided
that this validation was not possible.
However the decision was not made until the tentative examination of the feasibilityof using the roles had been undertaken by adding them to the indexing descriptions ofa small sample of reports. These, in fact provided examples of most of the objections
and difficulties we had already met in the earlier attempts to use roles. An exampleof the difficulties inherent in using the roles may be seen if we consider *a particular
document and some of the questions to which it had been judged relevant. In a document (1014) on the application of piston theory and the study of aeroelastic problems,
one of the themes indexed was calculation of panel flutter in supersonic flow by piston
theory. Some of the problems immediately raised by the addition of roles, taking
particular terms, were as follows:Panel is clearly a patient (role 9), but could conceivably be regarded also as a support or host in a process (role 5). Vibration (th& E.J.C. term for flutter) is undoubtedly an undesirable component (role 3) in the aerodynamic behaviour of the panel.

- 89 -

But this is a permanent feature of Flutter in the collection context and does not alter
from one document description to another. But roles are expressly designed to clarify
the local and varying relations between the terms of a particular description andperhaps this rules out role 5 from consideration. Flutter could also be regarded as
the primary topic (role. 8). However, firstly, role 8 is not strictly speaking a roleat all; it does not show semantic relations between the different terms of an index
description, only its subjective state as to a hypothetical reader; it i s , in fact, aweighting device. Secondly, it could be argued that piston theory is a primary topic(role 8). Perhaps both terms could be labelled 8, but piston theory could also be
regarded as an agent (role 10), although this would overlook the fact that it is not anagent of Flutter but of its analysis. Supersonic flow could be considered as an environment (role 5) or as a cause or influencing factor (role 6). If it is treated as (6) however, Flutter, which it affects, would be the factor influenced (role 7).
When the questions to which document 1014 was one of the relevant documents
are considered, the further difficulties in ensuring a match become apparent.
Question 97 refers to the Prediction of flutter on lifting surfaces. Flutter could
conceivably be given role 3 (undesirable component). or 8 (primary topic). It could
also be considered as role 9 in the sense that it is the object of analysis or predictive
operations. Two other questions, numbers 98 and 276, to which document 1014 isalso relevant, are on how flow characteristics or leading edge bluntness affect Flutter.
So now Flutter is an influenced factor (role 7), Yet another question, number 3, isabout aeroforces acting on high speed aircraft, and if Flutter is regarded as a kindof aerpforce, it may also be given role 8 (cause or influencing factor).
The general import of these considerations seems to be that the term can be forced
to play simultaneously a number of different roles in the same document according
to what the particular user is seeking, and that attempting to label them too precisely is liable to result sometimes in unjustifiable rejection of indexing descriptions
because they do not match exactly in the roles assigned them.