- 40 -CHAPTER 4
Indexing Procedures
The function of indexing in libraries and information retrieval systems is toindicate the whereabouts or absence of items relevant to a request. It is essentiallya time-saving mechanism. Theoretically, we can always find the relevant items by
an exhaustive search through the whole collection (assuming that we can recognize
what is relevant when we see it). Since this is economically impossible, the sizeof the store to be examined is reduced by classification, using this term in its verybroadest sense, i . e . , as the recognition of useful similarities between documents
and the establishment of useful document groups based on these similarities. So
documents, or document surrogates, are assigned to a limited number of classes
according to certain criteria, in particular, their subject content (although in machineindexing, utilizing complete text scanning, this 'limited number' can become verylarge - as large as the number of significant words used in the text). Search forrelevant items is made via these classes (which are classes of documents); only
those with a probability of containing relevant items are examined, and the rest
(hopefully the vast majority) are ignored. Clearly, we need to know as much as
possible of the nature of the classes to be recognised, and the degree to which they
allow reliable predictions to be made as to the probability of relevant items being
included in them.
Most library indexes, other than those to imaginative works (novels, musicscores, etc.) are aimed ultimately at the retrieval of subject information. Even
the great Author-Title catalogues, on which so much care has been lavished, serve
for the most part the function of a diagnostic classification, i . e . , an author's works
are sought in the first place because they are about a certain subject and his nameis a clue to locating it. The popularity of the author-title catalogue rests partly
on its precision in retrieval, Classes determined by authorship or title are mutually
exclusive; there is almost no overlapping, no ambiguity about them and requests canbe met with 100% recall and precision. But they are useless if the author or title
is not known and it is this situation with which IR is mainly concerned. So theclasses investigated by this project are those designed for searching by subject
prescription only.
There is one exception to this. Bibliographic coupling (including Citation indexing) establishes classes for much the same reason as author-title catalogues,
as an oblique way of getting at subject content. Papers which have cited item x are
assumed to have some connection with the subject of x. This particular device isdealt with separately in Chapter 7.
The terms which are used to express a request or a search prescription rarely
coincide exactly with the terms used to describe a particular relevant document;
this is likely to happen only at a relatively broad level, when a request may be answered by a treatise or monograph on the subject. For example, in the test Q. 93
read 'What investigations have been made on the flow field about a body moving
through a rarified, partially ionized gas in the presence of a magnetic field.* Twodocuments relevant to this question were1296 'Waves through gases at pressures small compared with magnetic pressure'1446 'Waves of a satellite traversing the atmosphere'
In both cases the match is very imperfect. It is made only by recognising that the-41 -

class prescribed in Q.93 can be adjusted in order to coincide at some points with
the index descriptions of the documents. The class 'rarefied, partially ionized gas'
must be seen to correspond or relate, after suitable manipulation, to 'gases at smallpressure' in the one case and 'ionosphere1 in the other. The class 'body' must be
seen to relate at some point to the class 'satellite'.
So a subject index must provide facilities for adjusting and manipulating its
classes; it must allow the index classes, examined to be expanded or contracted,
and in different directions, until a match with the search prescription is recognized.
Index language devices are the agents of this manipulation. They are devices whereby
class definitions may be adjusted to meet the requirements of different searches.
Index language devices
The index description of a document is a condensed (usually a highly condensed)
statement of the document's subject content; it seeks to convey succinctly what the
document is about. Its main, and sometimes only, constituent is the set of substantive
terms (lexical elements) which act as clues to the subject of the document. These
terms may be supplemented by some indication of the relations between them (syntactical elements), e . g . , by the addition of roles, or facet indicators (explicit or
implicit) or by such elementary syntactical devices as those of the Alphabeticalsubject catalogue. In a post coordinate index they are usually kept to a minimum,
enough to remove serious ambiguity but no more.
It seems reasonable, then, to assume, as the simplest possible form of index
description, a bare list of words, selected directly from the title and text of a document as being good clues to its content, and presented without any reference whatsoever to a control list for synonyms, related terms, etc.
The simplest way in which such a list of words could be used would be to regard
each word as defining one of the classes to which the document belonged, without
reference to the other words. Searches would then be made simply within theseclasses, separately. For example a document indexed as being about Wakes Satellites - Traversing - Ionosphere would be seen simply as a member of four different classes (the class 'Documents dealing with Wakes', the class 'Documents dealing
with Satellites', and so on). So a search on Satellite wakes would be made simply
by examining all documents in the class Satellites, and all documents in the class
Wakes. This is very similar, of course, to what a Permuted Title or KWIC index
does. Recall performance figures for this crudest of all forms of index language wereassessed in the first Cranfield project as 97%. Precision figures were not available
but it is certain that they were very low. It is assumed that all the keywords constituting the question are examined. If a selection were made, recall would probably
drop in so far as the exhaustivity of the searching would have dropped. The questionof exhaustivity and specificity of searching and indexing is discussed later.
Now will be considered the ways in which, by the use of various devices, thissimplest of all possible forms of indexing can be refined in order to increase its
capabilities for meeting all the demands which search prescriptions may make on
it. Such devices may be separated conveniently into two groups;
1, recall devices - those which, when applied to any existing class, increase thesize of the class in terms of the documents responding to the definition; e . g . , if
the class Bakelite is expanded by hierarchical linkage to include all Phenolic resins,
more documents are retrieved,

-42 -

2. precision devices - those which, applied to any existing class, decrease thesize; e . g . , if we coordinate Bakelite with Extrusion and eacamine only the classdefined by this simple relationship of intersection, we exclude those documents on
Bakelite which do not refer specifically to this operation.
The operation of these devices on the sort of simple index description describedabove can be seen by a consideration of the relations such a description displays
to the precise subject of the document concerned, and to the wider subject field of
the information store from which we may wish to retrieve that document or something like it.
An index description ay b^ c, d , e , f (where each letter represents a substantive
term or lexical element, e . ^ > % i 5 g f ^sets of relations:firstly, those internal to it, reflecting the local and temporary conditions peculiar
to the subject of the document described; e . g . , the fact that d-is the product, whereas
f_ is an agent of the process ia which produces it, or, the facfthat b qualifies £ while£ qualifies d, but that neither of these qualifiers is applicable to the object of "theother; or, more subjectively, that a, and b f rather than a id« or j[, represent the dominant theme of the document. These are,"broadly speaking",Ihe interlocking relations
between the substantive terms.
The second set of relations are those external to it, reflecting the more permanent pattern of relations in the wider field or subject area to which the document
belongs: e . g . , that ja is a species of x, or that £ is almost synonymous with £, orthat a represents one participle of a term (e.g.v Cooling) which may be usefully
related to another participle (e. g. Cooled) in the subject concerned.
The two sets of relations can be utilized to add precision to the original description (using the first set) or to expand the description by reference to the wider
relations (using the second set). In other words, they underlie the two groups ofindex language devices which will now be outlined briefly.
Devices which increase precision(i) Coordination - i. e . , the conjunction of two or more terms to produce a narrower class defined by the intersection; e.g. Shear and Flow to give Shear flow.
This is the most important device in indexing. Whilst it is commonly associated twith postcoordinate systems where it is implemented mainly if not entirely at the
search stage, it is equally fundamental to precoordinate systems; but in these, only
the products of selected coordinations are usually catered for conveniently.
(ii) Weighting - i. e . , the assignment to a term of a figure representing the relative
significance of that term in the total subject description of the document. So a term
which represents the central theme of the document gets a high weighting and one
which represents only a marginal element in the subject content of the document
gets a low weighting. If now a question is also weighted, i . e . , greater significanceattaches to one or some of its terms than to others, then the search may be directed
only to coordinations with that term or only to the same term when it has been given
a similarly high value in indexing. In either case, the class of documents retrieved
is made narrower.
(iii) Links - I.e. f indicating a particular connection between two or more terms
in a description where the lack of such an indication would produce ambiguity; e.g.,
if the same document deals with the hardness of copper and conductivity of titanium
a link between Hardness and Copper on the one hand and between Conductivity and-43 -

Titanium on the other would make it clear which property referred to which substance.
Clearly, a link must involve at least two terms.
(iv) Roles - i . e . , indicating the role or function of a particular term in an indexing
description. Sometimes a simple link is insufficient to remove ambiguity; e . g . ,Production of particle x by bombardment of particle jr. To link, say, Production
with x and Bombardment with £ could still allow the description to be interpreted
as Production by x, or Bombardment of £• The addition of a role-indicator makes
the relationship more explicit/ e.g. by labelling x as Product and £ as Patient (andpossibly a bombarding particle, z / a s Agent).
Devices which increase recall(v) Confounding synonyms - i . e . , accepting items indexed by x when searching for£, and vice-versa, where x and £ are regarded as synonymous.
(vi) Confounding word forms - i. e . , accepting items indexed by different forms of
the search terms, such as its singular and plural, participle and gerund; e . g . ,Injectant + Injected + Injection + Injectors. The most comprehensive operation ofthis device is where a stem or root is used to define the class and all words containing it are included in the class.
(vii) Hierarchical linkage - i . e . , accepting items indexed by terms which are in some
generic hierarchical relation to the search term. By this we mean terms which areeither subordinate to, superordinate to, coordinate with, or collateral with the search
term; e . g . , the class Cooling might be extended hierarchically to include the subordinate term Sweat cooling, the superordinate term Heat transfer, the coordinate
term Heating and the collateral term Radiation. This is a stricter interpretationof hierarchical linkage than is often used in the literature of IR, confining it to therelations between a thing and its kinds as distinct from numerous other relations
such as those between a thing and its parts, its processes, its properties, the operations performed on it, and so on.
It is perhaps necessary to note that hierarchical linkage is essentially a recall
device. This is not to say that it cannot be used in order to refine a question andgive it more precision; e . g . , an enquirer about to search the class Cooling might
be led to realise (by the hierarchical display of related terms) that he really wanted
Sweat cooling and would then narrow his search by confining it to this species ofcooling. This is almost as though hierarchical linkage were acting as a precision
device. But clearly, the question asked was wrongly put and the function of thehierarchy would have been to assist the question- programming. In testing devices,it must be assumed that the question has been accurately stated, otherwise a large
and disrupting variable will enter the test. Therefore such adjustments as the one
described cannot be considered, and hierarchical linkage must be treated as a recall
device only.
(viii) Non-generic hierarchical linkage. The device of hierarchical linkage which
has always featured prominently in subject indexing and is the central device in what
is generally known as 'classification' is the result of selecting one particular relationship (the generic one/ between a thing and its kinds) as the basis of various kinds ofclass definition. This raises the question: should we similarly regard the use ofother particular relations ( e . g . , b e t w ^ * thing and its parts, a thing and its properties) as constituting separate indeaanfc^Vices, each to be evaluated separately?

-44 -The importance of the genus/species, or thing/kind relation needs little explanation. Since Aristotle, it has been the central relation considered in traditional
logic. In information retrieval its use as an indexing device ('hierarchical linkage')is that it provides an automatic relevance network based on the inclusion relation,e. g., if laminar boundary layer is a kind of boundary layer, then documents on thelatter are likely to have some degree of relevance (possibly a high one) to questions
on the former. It is also the basis of the notion of 'specificity', which is a fundamental
parameter determining precision in index performance. It might be argued that the
non-generic relations displayed in all library classifications also possess an 'inclusion'quality so far as documentary classes go; e . g . , a pre coordinate class with the index
description Delta wing - Drag could be said to be 'included' in the class Delta wings
just as much as the true species Sweptback delta wing is included. But the inclusion
relation of the first example is weaker, in that it is a shared one; Drag on a delta wingis as much included in Wings-Drag as in Delta wings, whereas Sweptback delta wingis entirely included in the class Delta wing. That is to say, the non-generic relation
is essentially one of conjunction ('coordination') rather than complete inclusion.
Certainly, no other single relation rivals it in ubiquity or importance for retrieval, although collectively the other relations contribute significantly to class definition in indexing. In precoordinate classified indexes it is normal in questionprogramming tovmove from the terms of a category or facet, each one constituting
a hierarchy, to the particular term which gives rise to the facet; e . g . , to move from
Conductivity (of titanium) in the Properties facet of the class Titanium, to Titanium.
The object of systematically arranging classes in such indexes is to provide a permanent and constantly available mechanism for manipulating the classes in this way,
and not only by movement within a strict genus/species hierarchy. Classes are expanded or contracted by moving also frdm one category to another. For example,
in a question on compressor operation,the class examined may be expanded by movingfrom particular parts of the compressor (Blade, Shroud, etc.) to particular processes(Stage interaction; Stage stall, etc.) or to particular characteristics of these (Inlet
blade angle, Stall limit line, etc.)
In fact, the term 'hierarchical' is frequently used as a synonym for the processof subordination which is the essence of a precoordinate system. In this view of hierarchical linkage a chain such as Delta wing - Sweptback - Transonic speed - Low angleof attack - Lift is regarded as reflecting a hierarchy every bit as much as a true genus/
species chain like Metals-Titanium, or Aircraft-Heavier than air-Monoplane-with
Delta wing. It is possible to go even further and refer to the precoordination of an
Alphabetical subject catalogue (which, of course, subordinates some terms to others
in its subject headings, e.g. Aircraft-Design) as a hierarchical system (Ref. 21).
Even if this last extreme view of the term 'hierarchy' is rejected, we are left
with the fact that the identification of hierarchical linkage with the full range of relations displayed by a highly organized library classification system does not give us
a basic device which can be measured in the same way as can coordination, weighting,etc., but is a varied mixture of relations capable of defining classes.
In traditional library classification it is well established that any relationshipmay be used to establish 'subclasses' of a given class. In the sense that libraryclassification deals with classes of documents, and that documents on any aspectwhatsoever of a subject x can be regarded as belonging to some subclass of 'Documents on x», then Extrusion of plastics is just as much a subclass of Plastics as-45 -

Bakelite. But recognition of this is of little use unless the further step is taken oforganizing these subclasses according to their particular relations, in terms of subject content, to the class x . , i . e . , of recognizing that some are the properties ofx, some are its parts, and so on.
In a modern faceted classification these relationships are systematically displayed. All terms standing in the same relation to the original class are marshalled
together to constitute a category or facet of that class. So,
whatever concept
comprises the original class, all its Properties are assembled together, all its
Operations, all the Agents of these Operations, and so on. So far as they lend themselves to the process, the members within each category are organized in a hierarchy
and their relationship within the category is one of a thing and its kinds - kinds ofproperties of x, kinds of operations on x, kinds of agents of operations on x. But
hierarchical linkage in the strict sense, that is generic hierarchical linkage, is established between the terms within a category, not between individual terms from different categories, or between the terms of a category and the original class.
However, it is undeniable that in indexing and searching, classes are manipulated(i.e. expanded and contracted) by going outside hierarchical linkage in the strict
sense defined above. The two main paths pursued are those already indicated; firstly
to move from a term in one category of a class to one in another category of the sameclass, e.g. Separation see also Boundary layer control (where Separation belongs to
the Process facet of Boundary layer and Boundary layer control belongs to the Operations facet). Secondly, to ijaove from a term in a category to the original class giving
rise to the category, e. g. Blowing see also Boundary layer (where Blowing is anoperation designed to accelerate the flow in the boundary layer, and belongs to the
Operations facet of Boundary layer).
Both the above types of connection imply a more or less definite subject area inwhich the terms in question stand in some categorical or facet relation. Another typeof connection is sometimes recognized, between terms which come from quite distinct
areas and which are therefore not considered to have such a facet relation. The 'phaserelations' of Ranganathan.are one example, and some of the terms connected in syndesis constitute another. For example, a thesaurus might link Automatic control
theory with Aerodynamic stability. Generally speaking, the view that this constitutes
a quite distinct type of relation assumes a relatively arbitrary map of the field ofknowledge in which subjects are assigned to one conventional class or another, suchas those found in a general classification. It does not correspond to any fundamental
relation between the terms which, if they have any connection at all, can be fitted intp
the framework of categorical or facet relations. For example, Control may be viewed
as a term in an Operations facet of the subject Aerodynamic stability.
It is not feasible to consider syndesis ( i . e . , the adjustment of classes via a systemof linking references) as a device in itself since it clearly uses a mixture of several
quite different relations in indicating its further classes. Neither does it seem particularly profitable to make a rigid distinction between the two types of non-genericrelations above since it is likely that they will overlap from time to time; e . g . , Blowing
may well occur in a general Control Operations facet rather than be subordinatedsolely to Boundary layer; in which case the linkage between Blowing and Boundary
layer exemplifies the first type, not the second.
Also relevant to the question of whether these relations rank as discrete devices

- 46 -

is the fact that they are rarely, if ever; used alone; whilst all the devices given
earlier may be thus used (at least in a post coordinate system), the non-genericrelations are invariably associated with hierarchical linkage, whether this is via
a classified index or a syndetic network of connective references. In a thesaurus,
for example, generic hierarchical relations and synonym relations are often indicated separately, but individual non-generic relations are never recognized separately.
We conclude that there is nothing in the practice of indexing to suggest that aseparate evaluation of each non-generic relation is necessary, but the collective
contribution to indtx performance of these relations compared with the contributionof generic hierarchical linkage is a matter of some interest, and it seems reasonable
to group them together as a comparable device. It may be noted that generic hierarchical linkage is itself an aggregate of several particular relations, just as thisgroup i s . The problem is discussed further in the section on Concept hierarchiesin Chapter 5.
(ix) Bibliographic coupling is a device for extending a class x (representing the
subject of a particular document cj) by accepting all, or some of the documents whichhave cited £:' or, by accepting all documents in a particular universe which have acertain number of citations (6 or 7, say) in common with q.
(x) Associative indexing by machine (telumpfi^, etc.) The possibilities of automatic
indexing now being explored by a number of investigators rest mainly on the assumption that classes useful for retrieval purposes can be established on the basis of thestatistical characteristics of the index vocabulary (which may in fact approximate to
the complete texts of the documents concerned). By using such features as the frequency of occurrence and co-occurrence of individual words and of particular wordclusters, their position in the text, their relative frequency compared with a standard word-frequency list in the subject area concerned, and so on, associations
between terms are established which then form the basis of search programmes.
The criteria defining the classes to be examined are thus quite different from anyof those listed above and therefore the procedure constitutes an indexing device inits own right.
How far it might be feasible to distinguish particular procedures ( e . g . , the useof one statistical technique rather than another) is as yet uncertain. In particular,
the purely statistical methods are in some cases replaced by methods using linguistic analysis, and insofar as thes^ must overlap the 'semantic 1 devices already described (confounding of word form*, hierarchical linkage of various kinds) they may
not merit the status of a discrete and unique index device.
(xi) "L'Unitfe" system described by te Nuyl (Ref.22) is a somewhat exotic device
whereby a reduced vocabulary is established in a quite mechanical way by lumping
together all the terms in a given sequence of pages in the Concise Oxford Dictionary
and treating their aggregate as a single class. As is the case with all drasticallyreduced vocabularies, it is argued that the theoretical absurdities which might arise( e . g . , the appearance of documents on Acne in a search for Aconite, or on Conductivity in a search for Cones) do not arise in fact, since subsequent coordination
eliminates them.
Any reduced vocabulary may be regarded as a recall device, in that it implies
enlargement (by coalescence) of the classes which are formed initially by the individual index terms assigned to a document. Usually, reduced vocabularies are formed

- 47 -

by a mixture of the devices already described - by hierarchical linkage, by confounding word forms, etc. "L'Unite11 uses none of these, although the last-mentioned willin fact normally be a prominent accompaniment of its class definition, since different
word forms will usually appear in the same alphabetical cluster. So it must be admitted that it forms a discrete index device in its own right,
The above eleven devices may be compared with the list of techniques for controlling index languages given by B. C. Vickery in his book 'On retrieval systemtheory1 (Ref. 9 ) (see Chapter 1). Of those listed above, three devices, namely Bibliographic coupling, Associative machine indexing, and L'Unite, were not mentioned
by Vickery. The others include all the techniques given by Vickery, since a numberof these were variants of the more broadly defined devices above.
We have tried to distinguish the basic device itself, as a method of class definition,from the different ways in which it might be implemented in different index languages;
The latter may be regarded as different amalgams of the various devices, with further differences resulting from the various methods of file organization.
The different ways in which coordination is applied in precoordinate and post coordinate systems, have already been mentioned. The fundamental difference isthat in the former a limited number of coordinations are made and the resultant compound, headings are then filed in linear order, and rules observed (as to citation order,etc.) to allow determination of the exact position of any particular combination. Thisdifference has repercussions for the other devices. Even in a largely single-entry precoordinate system, for example, 'weighting1 of an elementary kind is implicit in
the restrictions placed on the number of entries which can be recognized. Links arefundamental to a precoordinate system by 'partitioning1, e . g . , separate entries
would be made for Copper-Hardness and for Titanium-Conductivity.
Roles are indicated in a precoordinate system by citation order or by explicit
syntactical devices; in a faceted index an index description such as
Wings - High aspect ratio - Drag - Low angle of attack
conveys by its citation order of Thing (Wing, etc.) - Property (Aspect ratio, etc.) Process (Load, Drag, etc) - Condition (Aerodynamic parameters, Angle of attack,etc.) that High specifies Aspect ratio and Low specifies Angle of attack. In an Alphabetical Subject Catalogue, a similar function is served by such headings asChildren in art,
Pressure vessels - Heat transfer,Acceleration - Psychological effects.
Synonyms are treated with varying strictness of interpretation in different systems.
Often, it is a reflection of the degree of specificity sought, as when one system distinguishes Potential flow from Irrotational flow and another confounds them. The
problems of synonymity occurring at the level of multiple term descriptions raises
a particular problem for post coordinate systems, since it implies a degree of precoordination at some point in the systein; e.g. a Ground effect machine is a synonym
for Air cushion vehicle, although there is no synonymity between the individual constituent terms.
Confounding of word forms may be implemented at the indexing stage, via a controlled vocabulary ( e . g . , Conducting see Conduction), or by search rules (e. g., accept
Conducting + Conduction + Conductor). In a precoordinate system, where the different forms are separated in different categories, confounding may be possible onlyat the search stage; e. g., by consulting the A/Z index of a classified index and observing the variant forms used.

- 48

Hierarchical linkage may be implemented in a post coordinate system by generic posting,by coding based on a form of semantic factoring as in the WBU system,
or by search strategies based on a classification schedule or thesaurus. In a precoordinate system it may be achieved to a large degree by the file arrangement, asin the systematic juxtaposition of related classes in a classified file; it may be achieved
fragmentarily by inversion of subject heading in an Alphabetical subject catalogue( e . g . , Drag, Base; Drag, Form; Drag, Induced; etc.) or by search strategies basedon a syndetic network of see also references*
Measuring the performance of index devices
In order to establish recall and precision performance figures for the different
devices, both singly and in various combinations, it was first of all desirable that
we established as far as possible figures for indexing in which none of the devices
was operating. Then it would be possible to determine the impact on these figuresof the introduction of each device in turn. This assumes, of course, a test collection
and a set of questions to be put to it, where it is known just what documents are relevant to each question, as described in the previous chapter.
Performance figures for an funindexedr collection seemed to imply a situationin which the complete text of each item in the collection was searched for each question. This would have been too tedious an operation (although something like it,except that it was on a small scale, using computer facilities, has been described by
Swanson (Ref.18) ). The alternative which we decided to take, was to use, as thebase situation, one in which the simplest known indexing device was used and tomeasure the impact on this of all the other devices. This simplest device was taken
to be that of condensation of the full text into an index language consisting solely of
the 'uniterms1 thrown up by the title and text of the. document itself, quite uncontrolled by any prior index language.
So the first step was to establish, by the indexing of the test documents, a crude,elemental index language from which all the other languages (each one characterized
by the addition of a particular device or aggregate of devices) would be derivable.
Before this could be done it was necessary to provide for the control of two major
parameters in indexing, exhaustivity and ^specificity.
Exhaustivity and specificity
Exhaustivity in indexing refers to the degree to which one recognizes (i.e. includes
in the index descriptions) the different concepts or notions dealt with in a document.
Specificity refers to the generic level at which these concepts or notions are recognized. For example, suppose a report has as its main theme the subject 'Drag on
swept wings at jiigh subsonic speeds 9 . If one neglects,, for the time being, the various
subsidiary themes which are also dealt with, this report may be said to deal withthree concepts - an aerodynamic characteristic, an aerodynamic structure and aflow condition. If these concepts were described in the above fashion in the index
description, this latter would be exhaustive but not specific. If the description consisted only of Drag - High subsonic speeds it would be neither exhaustive nor specific; for whilst the terms retained are specific, the absence of any reference to
Swept wings implies that the subject deals with aerodynamic structures in general(some structure i s implicit, of course) and this is less than specific, since to be
this a description must be exactly coextensive with the notion represented. Therecan be no reduction in exhaustivity which is not a reduction in specificity; but thereverse does not hold.

-49 -An exhaustive and specific description (either in indexing or in question formulation)i s o n e which allows no further qualification or refinement - it is a completely precise
statement of the class concerned (or classes, if the terms are considered individually).
Hhseems clear that such a description should include not only the substantive terms
or lexical elements (which are the essential and often sole constituents of most index
descriptions, at least for post coordinate systems) but also the full range of interlocking relations, or syntactical elements, which convey the exact relations between
these terms in that particular description. To take a rather farfetched example,
another report might refer to a high wing at subsonic speeds and unless, in the firstexample, High is interfixed or linked with Subsonic speed the two different subjects
are not clearly distinguished. Unless we are to recognize these syntactic elements
as a third parameter in the precise description of a document, they must be regarded
as elements in exhaustivity and/or specificity. In the great majority of cases they
do not refer to the generic level of substantive terms but reflect non-generic relations;they constitute 'relational1 terms, analogous to th£ substantives. They are used assuch in a few indexing systems and theoretically at least can themselves display
varying generic levels; e . g . , Influence could be replaced by the more specific Harmful Influence. It would seem, then, that these terms reflect both exhaustivity andspecificity, but more often the former. Only when the relation is explicitly a genericone (as can be the case, for example, with Parradane»s appurtenance operator, Ref. 23)
can they be said to determine specificity.
Exhaustivity of indexing
Recall devices cannot be fully tested unless the indexing on which they are tested
is exhaustive; jotherWise, loss of recall at any point might be attributable to a lackof exhaustivity*; rather than to the device concerned. So maximum exhaustivity in indexing was attempted, at least as far as substantive terms (lexical elements) wereconcerned. At the same time, since it was clearly desirable to measure the effectof varying exhaustivity on different devices, it was necessary to note during the indexing which terms would in fact have been omitted if any level less than complete exhaustivity had been acceptable.
This, problem was very conveniently solved by using the figures assigned to terms
as a weighting.device as indicators also of which particular terms would have been
accepted at different levels of indexingexhaustivity. For the highest level of exhaustivity all terms would be acceptable, whatever their weight. For the lowest level of
exhaustivity only those terms given the highest weight (i. e., those terms which would be
regarded as essential even in relatively superficial indexing) would be acceptable.
In the result, on average, 31 terms were used per document, with 3 levels of
weighting. (6, 8, 10). If only those terms weighted 8 or 10 were counted, it represented an exhaustivity level of 25 terras per document and if only those weighted 10
were counted it represented 13 terms per document.
Of course, 'complete exhaustivity' is a relative term here; strictly speaking only
the use of the full text of the document including diagrams, tables and graphs, constitutes completely exhaustive indexing. But whilst the economics of mechanised
aids in indexing may eventually make this feasible and its testing desirable, thedegree of exhaustivity represented by 31 terms per document was thought to be areasonable approximation to what would be regarded, for documents in this particularsubject area, as extremely thorough indexing. The problem of syntactic elements('relational terms') as an element in exhaustivity will be dealt with later.

- 50 -

Specificity of indexing
Precision devices cannot be fully tested unless the indexing language on which
they are tested is of maximum specificity. For example, a question on Ellipticalcylinders can only be matched Specifically if,'^^ whenever that concept appears duringindexing it is represented by the exact description IJUiptical cylinders and not by
a more general term such as Cylinders alone/ Ifthe indexing is not specific in thefirst place, there is nothing the searcher can do to improve precision by altering
his search programme.
At the level of substantives, or lexical elements, specificity was fairly easy
to achieve, since, by adhering closely to the languageof the document and indexing
exhaustively, it was reasonably certain that the specific subject of a theme or concept
would be brought out. Even if the author used i more general term in the title orsummary, as was often the case, the specific term would nearly always appear somewhere in the text. For example, the title might refer to a 'Laminar boundary layer*,
the seminary to an 'Incompressible boundary layer 1 and the body of the text to a
'Steady, laminar, incompressible boundary layer'; the indexing would give Steady,laminar, incompressible boundary layer.
Effect on indexing procedures of methods of measurement
Having provided for the control of the major parameters of exhaustiyity andspecificity, the problem arose of how the different devices might be added, one byone, to the basic natural index language, so as to allow for their measurement.
Several possible methods of proceeding now presented themselves: :(1) To make one index completely devoid of any devices, and concurrently, to makea number of separate indexes, each one embodying this first index modified by a single
device, e.g. one index in which the varying word forms o f a term were confounded,
another in which hierarchical linkages were established, etc.
(2) To make a device-less index and measure the impact of devices entirely by variations in search programming; e . g . , the result of confounding synonyms could bemeasured simply by programming a search for 'Disturbance' as 'Disturbance + Perturbation1 whereby the expansion of a class is achieved simply by making a sum of
the constituent parts of the expanded class. Similarly, measurement of the effect ofconfounding word forms could be effected by programmes such as 'Injecting + Injection +Injector . . . •In comparison with (1), this method, obviously, would be much less laborious
clerically, even in thfr case of hierarchical linkage. It might be thought that this
could be best measured by constructing a classified index: but the latter is ah amalgamof several devices and the measurement of strict hierarchical linkage in isolation ismeasurable quite effectively by such programmes a*: W ^ e + [(Wave x (N + Standing +Blast 4- ShpckO to expand an initial class like 'N Wave' to the generic containing class;'Wave'.
Of course, such search programmes required the compilation of code dictionaries of synonyms, of wprd-forms, of hierarchies (i. e. of classification schedules^. But
the indexing itself, so far as these recall devices were concerned, could be done
without any regard to these devices whatsoever, since the relations concerned did

-51 -

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tkQti depend on the context of the individual report being indexed but on the context of
the index language as a whole; in other words, the relations w'ere paradigmatic rather
t&an syntagmatic. It may be noted that all the devices measurable by Method (2) arerecall devices - i . e . , devices for elaborating or expanding the classes given in theipdex description of a document.
(t3) To incorporate particular devices in the original indexing but in such a way as tomake them detachable when required; e . g . , to attach weights to terms which could
be counted when figures for weighted indexing were required but ignored for unweighted
indexing. This method, also much less laborious than Method (1), would be particularly
appropriate for those precision devices which depended on the context of the individual
report being indexed, and which could not therefore be measured by Method (2).
It was finally decided that the indexing proper should be done on the basis of
Method (3); that is to say, the indexing would be basically post coordinate, and take
into account only the precision devices of weighting, links and roles (whilst observing
a high degree of exhaustivity and specificity). Method (2) was to be used in themeasurement of the recall devices of Synonyms, Word-forms and Hierarchical linkage(generic and non-generic). Associative indexing could not be measured within theconditions above, but it was hoped that the indexing would be sufficiently exhaustive
to allow some tests of associative techniques to be made by other investigators.
Te Nuyl's device was also ignored at this,point, since it was clear that our indexing
language could always be translated into dictionary-based clusters when necessary
for measurement by Method (2). Bibliographical coupling, since its classes are notdefined by subject descriptions; required quite separate measurement, and is discussed
in Chapter 7.
The major precision device of coordination is, in a post coordinate system, purely
a search device and its measurement does not fit exactly into either Method (2) or(3). It is perhaps necessary to mention at this point that post coordination in itself
does not constitute an indexing device. It is essentially a method of recording subject
descriptions in a physical form which allows equally freef access to whatever combinations of terms are requested. A precoordinate system, on the other hand, allows
direct access only to certain selected combinations of terms. Other combinations
constitute distributed relatives and access to them i s to this extent made less convenient (although it is by no means forbidden, or made impossible, as is sometimes
suggested). But the class defined by coordinating two or more terms is exactly thesame, whether the operation is performed at the indexing stage (precoordination) orat the search stage ( post coordination). The relative convenience with which accessto such a class is gained was not something with which this investigation was concerned.
The form in which the indexing was recorded is best shown by Fig. 4.1 whichshows the index sheet for document 1590. Author and title details were printed on
the sheet. The indexer then analysed the document in four stages: firstly, 'concepts'
were distinguished as a first-stage interfixing device Olink'); these are not easily
defined (and this must be recognized as a theoretical weakness) but their practical
function was reasonably clear. This was to remove the first level of vagueness andambiguity inherent in words taken singly, by not accepting adjectival forms alone
but only in conjunction with the terms they qualified. So terms which in isolation are
weak and virtually useless as retrieval handles were given the necessary context;
such terms as High, Number, Coefficient, Main, Trailing, Angle, Aspect which inpractice do not form classes for which requests are made, appeared in conjunction

- 53 -

with other terms, to produce meaningful class terms - e . g . , High subsonic speeds,
Mach number, Pitching moment coefficient, Main wing, Trailing edge, Low angleof attack, High aspect ratio. Not only 'weak1 terms were combined, however, for
the interfixing function of concepts often produces phrases whose constituent terms
are quite potent, index-wise even in isolation. For example, Cruciform wing, Circular body, Tail fins, Span loading theory, Force divergence Mach number, Wing
vortex field, Rectangular wing surface, Wind tunnel wall. Such combinations makeit clear, for example, that in the one document Surface relates to Rectangular wing,
not Cruciform wing; that Mach number relates to Force divergence rather than tosome other phenomenon; that Low relates to Angle of attack and High to Aspect ratio(and not vice-versa).
Or, as in Document 1590 that Distribution relates to Velocity
and Ratio relates to Total Pressure, and not vice-versa.
Secondly, the concepts were now grouped into a second-stage link device inorder to display the distinct 'themes 1 iiito which the document could be partitioned.
'Partitioning' of a document is a well-established procedure in traditional precoordinate indexing and is often referred to as analytical cataloguing . Owing to theexigencies of space in the precoordinate index, such analysis is usually confined toitems in which the constituent chapters, sections; etc. can stand alone; examples
are symposia of various kinds, festachriften, and collections of plays. In such cases,'standing alone' could be interpreted almost literally in the sense that each theme isdealt with in a distinct, self-contained physical section of the document. In suchpircumstances, partitioning could allow greater recall (resulting from greater exhaustivity of indexing) with almost no loss of precision. This was rarely the case in
the aerodynamics reports constituting the test collection. In these, a particular concept might run as a thread throughout the document, appearing at different times indifferent contexts. So themes were not necessarily, or usually, mutually exclusive.
This diffusion of various concepts throughout a document seems to be an importantcause of many problems in retrieval and particularly that of the inverse relation
between recall and precision; for a document whose index description contains all ormost of the terms of the question prescription may yet feature those terms in an unacceptable pattern.
The example in Fig. 4.1 (Doc. 1590) demonstrates the salient features of the twostages of linking embodied in the indexing. To economise in the writing down of
themes, the concepts were labelled with lower case letters and only these appearedin the themes. Where the relationship between concepts appeared to be potentiallyambiguous, it was indicated in an elementary fashion by verbal quasi-role devices
such as 'effect of1, 'by means of or 'use of. So the first theme of document 1590
is to be read as Axial flow compressor - Stage performance - Effect of Stage characteristics - Use^ of Test Data - Analysis.
Normal practice was to give as the first theme (or first few themes where necessary), the general subject of the document considered as an integrated whole. Subsequent themes would then bring out the particular subjects which made up the whole.
In document 1590, for example, themes A and B jointly represent a formal statementof the title, with the addition of Test Data and Analysis. It also demonstrates a fairly
common situation whereby the title provides a reliable and succinct statement of the
document's general theme.
The third step in indexing was to give weights to the concepts (and subsequently
to the terms) - i . e . , to allocate to each concept a value indicative of its relative importance in the document. Such a value can be regarded as an assessment of theprobability that, should the concept concerned happen to be the subject of a question,

- 54 -

the document indexed would be of some relevance to it. It might be argued that allindexing which involves a selection of terms to describe a document (and that is aU
indexing - even the use of full text by a computer, with deletion of articles, prepositions, etc.) implies weighting in that the use of a subject term indicates a reasohable
probability that the document is of some relevance to questions on that subject whereas
the rejection of a term indicates that the probability is low or non-existent. Weighted
indexing extends the range of values from two (worth using, not worth using) to whatever number of different weights are recognized. For example, if an index description contains the terms £ b £<d £, and weighting is assigned to each term in the scale
3. Most important terms
2. Less important terms
1, Least important terms
If a, b and £ are now each weighted 3, while (i and £ are each weighted 1, the implication is that the probability of the document being relevant to a question £ b £ is thatmuch greater than the probability of its being relevant to a question on ci £.
It has already been noted that the weights given to terms could be used as thebasis for measuring exhaustivity i . e . , when a figure for a high level of exhaustive
indexing was required, weights could be ignored and all terms regarded; when afigure for less exhaustive indexing was required, those terms which had low weights
could be ignored and treated as though they were not indexed. It should be noted,
however, that the use of weighting as a measure of. exhaustivity is purely an evaluation
technique and plays no part in normal indexing, for in the latter, weighting only
comes in as a device when it is applied also to the question, Then a question termwith a given weight will accept as a match only those index terms which have the same(or a higher) weight. This procedure inevitably alters the boundaries of the classes
defined in searching and proves weighting to be an independent index language device
and not just a reflection of exhaustivity.
The rejection of an index term as irrelevant is now performed at the search stage,
whereas exhaustivity of indexing is decided, of course, at the indexing stage. For
example, suppose a question containing terms £ b £ d £, and a relevant document which
has been indexed with weights as a? b* ci1 £ a g 2 etc. If we were simply measuring theeffect of exhaustivity, we would say there was a match of four terms ( a b d e ) whenindexing was fully exhaustive (all weights accepted). If terms with the lowest weight(1) are now ignored, the match is reduced to three terms (£, b, £); if only the highest
weight of terms is accepted (i. e. the lowest level of exhaustive indexing), then thematch is reduced to two terms (£b). Here we have been using weighting purely asa measure of exhaustivity of indexing, but to consider its effect as a precision device,assume that each term in the question is weighted,and that the search specification
is now £ 3 b5 & d1 rf The term £ is rejected because it does not appear in the indexing; £ is rejected because the search requirement is for a term with a minimum
weighting of 3 , whereas in the document £ has been indexed with a weight of 2. Thisnow means the class accepted, has altered to abci, whereas with variations of exhaustivity it was respectively £ b^ d £, £ b^ £ or £ b^.
As to the problem of how to weight, two general approaches seemed possible andboth were made. Firstly, for three hundred documents, weights were assigned on aquasi-statistical basis, dependent on the sections of a document in which a term appeared. If a term appeared in the title, the summary or abstract, the introduction,
the body of the text, the conclusion, and the list of cited references, it received a

- 55 -

weight of ten. If it failed to appear in any one of these positions its weight was reduced
by one point for each failure to appear. Since the indexing was strictly according to thenatural language of the document this meant that all terms received some weight insofar
as no terms were used which did not appear somewhere in the document.
Secondly, for the rest of the collection, weights were assigned subjectively. In >most of the literature on weights, notably the pioneering articles by Maron and others(Ref. 24), weights were assigned subjectively to individual terms. This proved unsatisfactory; e.g. in a document in which 'Low aspect ratio wings' constitutes a central theme, can the single terms Low or Aspect or Ratio possibly be regarded as
crucial in themselves? The significance of a term in a document is very often lost if
the term is robbed of its context. So weights were assigned to concepts and the individual terms within a concept received the weight given to that concept.
A range of six different weights was again adopted. This time it attempted to
combine a measure of the importance of the concept in relation to the total messageof the document (which is another way of referring to the documents probable relevance to a question entailing the concept) with an assessment of its significance inretrieval terms, i. e . , its potency as a retrieval handle in the particular collection
indexed. The subject significance was measured by reference to a trio of values;
these assume that a document can normally be regarded as consisting of its integrated
subject as a whole (its main theme), together with one or more subsidiary themes,
which may vary in importance from quite major component themes to quite minor,
marginal themes. This assumption is a simple extension of the analysis into themes
already described as •partitioning1. According to the status of the theme in this scheme
it received weights between 9/10, 7/8 or 5/6:
Weights
9/10
7/8
5/6

For concepts in the main general theme of the document
For concepts in a major subsidiary theme
For concepts in a minor subsidiary theme.

When assigning the weights to the individual terms, the higher weight of the pairassigned to the concept concerned was used if the term was considered to be a very
potent one; potency here was regarded as a mixture of word frequency in the totalcollection (indicating roughly the generality or otherwise of the request in the context
of the particular collection), and 'concreteness*, whether the term was likely to berequested as the focal point of a subject or whether it was too vague to be the object ofa direct and separate request. For example, in Document 1590 (Figure 4* 1) theconcepts of Themes A and B were each allocated the top weight; the individual terms
which were considered potent (e.g. Stage, Matching, Compressor, etc.) received aweight of 10; Flow (on the grounds that it was a very common term) Test, Data, Analysis(on the grounds of vagueness) received the lower top weight of 9.
One small point to be noticed is that an individual term was always given theweighting of the more heavily weighted concept if it appeared in more than one concept. In the example, concept m is Idealised compressor, and the concept is given
a weight of 8. However the term Compressor has previously occurred in concept c^,
Axial flow compressor, for which it received a weight of 10, and this it retains.
The fourth step in the indexing was to write out the terms individually and attach
the weights to them as explained above.

- 56 -

The fifth step was to assign roles to the terx$s. At this stage a temporary failureof plan has to be recorded. If roles were to be assigned/ it was obviously desirable
to assign them at the time of indexing. Preliminary analyses were made of a numberof complete indexing descriptions in order to establish the major variations of roleoccurring amongst the terms 1UU! the degree to which this might lead to ambiguity.
It was stated earlier that the main function of roles would seem to be the removalof a certain type of ambiguity beyond the power of simple links to remove. Links, it
should be remembered, simply assert that some relation exists between two or moreterms - between ja and b, say, rather than a and cv A role states what the relation is.
From the preliminary analyses, it became apparent that the roles developedby Western Reserve University, American Institute of Chemical Engineers and by
DuPont were not appropriate, designed as they were for subject areas (chemistry
and applied chemistry) in which the appearance of the same term (e.g. a material)in significantly different roles was a fairly frequent experience. The Cranfield investigation of the W.R.U. index had already shown some of the drawbacks associatedwith roles, even in those fields for which they seemed particularly appropriate, andour analyses confirmed these, A major problem was the fact that when a term plays
one particular role in an indexed document, it does not necessarily make that document irrelevant to a question in which the same term features in a different 1role:for example, a document on the 'Use of mufflers to control the sound of jets suggests
the roles: Muffler (Agent of operation). Control (Operation). Sound (Product),
Jet(Cause). If a question were now asked on the 'Effect of mufflers on jets9 the role of
Muffler might well be designated as Influencing factor (cause) and that of Jet as Thing
affected; the relevance of the document to this question would be obscured by thedifferent roles assigned to the otherwise matching key terms. Such a situation implies that several quite different roles might be acceptable in searching; but this comes
dangerously near to negating the whole idea of roles. Such* in fact, was the situationin the W.R.U. test, when the roles were frequently ignored in W.R.U. search programmes, presumably because of awareness of the danger of demanding too closea match.
An alternative plan which seemed to be suggested by the kind of example given
above was that roles should not be assigned purely according to the relations between
the terms in the document or question concerned (their syntagmatic relations), butalso according to a wider picture of the relations between the terms in a subject area
(their paradigmatic relations). It is generally recognized that the organization ofterms into facets is closely related to the provision of role indicators. In special
faceted classifications it is not uncommon to find a term appearing in more than one
facet, the difference being due to the difference in role played; e.g., in a classification for pharmaceuticals, the same substance might appear as a Product, a
Substance Extracted, an Agent of a reaction, or as an Agent of an operation. In sucha system, where prior analysis of the terms of vocabulary has been undertaken, themore enduring relations (that a problem or by-product in the propagation of jets isthe production of noise, which demands control, and that mufflers are one agent ofcontrol) would be recognized and the fortuitous alteration of roles suggested in thequestion would not have been allowed to obscure the situation.
Consequently, a set of roles was developed along these lines so that they closelyreflected the categories which would be distinguished in facet analysis of the field.
But trials of these (i.e., examination of a number of indexing descriptions in order
to see whether ambiguities would be removed by the roles) showed that they left untouched what was probably the commonest problem of ambiguity in the vocabulary$- 57 -

being developed. This was the problem of knowing what terms were qualified bywhat adjectives. The example of 'High aspect ratio wings at low angles of attack'has already been cited. Without some interlocking device (links or roles) this document description would respond to a question on 'Low aspect ratio wings', or 'High
angles of attack1. Similarly, an index description 'Transient aerodynamic heatingof external loads' might respond to a question oh 'Transient loads' or 'External
heating', and 'Compressible flow over an adiabatic wall' might respond to a question
on 'Adiabatic flow'. However, the situation i s typically one which is solved by links
rather than roles, since the type of relation is not in disptite, only which of the two(or more) terms are to be linked.
If such modifiers were to be regarded as roles the only solution which would
remove all ambiguity would be to give them the same role as the term they modified,
and then it would work only on the assumption that links were used as well. This
was therefore tried (using such roles as Specifier of agent, Specifier of structure),
but only at the cost of duplicating the linking function already performed by partitioning
and interfixing.
One of the few situations,in the literature indexed in which a concept (rarely asingle term) might feature in a role which appears to differ significantly from its
usual one is that in which a body of data is used as an agent in the investigation ofsome other phenomenon (to which it therefore takes second place in the document
concerned). For example, in Bl204,Two-dimensional airfoil section data are used
as a means of studying high subsonic speed characteristics of swept wings. But
closer examination throws doubt on the necessity for using a role even here; had thesection data been referred to in some other way (as a parameter influencing aerodynamic behaviour, say) its essential relation to the primary subject (swept wings)
would not appear to have been altered.
It was stated earlier that in transferring concepts to themes (the second step inindexing) the exact relationship was sometimes indicated for clarity. The relative
infrequency with which the need to do this arose (and Document 1590 was not typicalhere) was itself a warning that the field of high speed aerodynamics was not likely to
be very fruitful in the evaluation of roles as a device. This impression was reinforced by our preliminary analyses. Since, then, any measure of roles was likely
to reflect an uncongenial and unresponsive test environment, the very considerable
effort involved in developing a set of appropriate roles, applying them and measuring
the impact, seemed of doubtful worth. For this reason the preparations for theevaluation of roles as a device were temporarily abandoned.