IllS Y N T A X

III o 1

Input syntax

Syntactic information in document descriptions may be implicit
or explicit* In treating keyword strings or compound subjects asunits? we rely in matching on the presence of relations between theelements involved, though these are not defined. Alternatively we
may make the relations explicit? minimally by ordering or bracketing?
more fully by specifying particular relationships. Analysis may
accordingly be intended simply to identify word strings which function
as units? or to spell out the relational structure of the input texto
In the first case partial parsing procedures may be adequate? in thesecond full analysis may be required0
Syntactic analysis may also be exploited to identify text itemswith specific syntactic functions,, on the grounds that these are key
content indicators.
Most automatic syntax analysis in document retrieval comes under
the heading of partial parsingQ Few projects have attempted fullparsing* This is more usual in question answering systems*

IIIclol

Partial parsing

The lowest level of partial parsing is represented by the use of
specific? very restricted clues as aids to the selection of particular
itemso Borkowski's 1970? 1973 work on case citations in legal texts?
in which "Vc9' is used as a trigger to pick up related items? is an
example,, A similar approach using templates of the form 'from o a a
to ooo° to identify dates is suggested by Weiss 1969o
The next level is characterised by the use of a wider range ofclues to process complete sentences rather than merely to selectcertain items. For example we may use punctuation marks? prepositions
and conjunctions to divide sentences into strings of words which may
be treated as units. Titles naturally lend themselves to this treatment? and it has been attempted in Armitage's 1967? 1968? 1970 workon articulated subject indexesc This will be discussed latero
In this case there is no attempt to characterise the phrase
structure of the separate strings identified*, Clarke and Wall's
early project? 1965? went a step further in parsing to identify
phrases? and specifically noun phrases? though they did not try to
relate the phrases to one another to produce an overall sentence
characterisationo Carroll 1973 reports without details the use ofa text analyser to identify noun phrases for profiles0 Since theobjective is limited? the analysis is presumably fairly simple0

3.1

The procedures described by Klingbiel 1973a,b are intended tobe applicable to the vast quantities of material received by the
DDCo They have already been tested on millions of words of abstract
text,. The syntax analysis routines select wcrds or word strings forpossible use as index descriptors0 Text is segmented, for example atpunctuation marks0 Individual words are checked against a dictionary,and sequences of syntactic categories for selected words are vettedby reference to a format dictionary of 76 entries , Word strings
passing the format test are offered as candidate index terms forhuman reviewQ The examples of output given in Klingbiel 1973b arequite plausible, and the scale of the whole operation is very
impressive* However retrieval procedures have not yet been developed.
The next level is reached where a sentence is substantially orcompletely parsed, but the overall refinement of the structuraldescription is not very great because only limited information is tobe selected for use in document description. Approaches of thissort verge on full parsing „One project of this type is Hillman0s 1968, 1969, 1973„ It isdifficult to be certain of the details, since there seem to havebeen some changes in the techniques usedc Hillman*s approach isfairly sophisticated % it is designed to identify noun phrases, which
are assumed to be the main content bearing text items, and also theirrelationshipso A relatively simple phrase structure parsing of asentence is therefore processed to discover the logical relations
holding between noun phrase units, chiefly by looking at verb environments.
The analysis output is a set of canonical components representing
propositions expressing relations linking noun phrases, to which theoriginal sentence has been reduced0 This relational information is
not, however, preserved in document descriptions, but is merely used
to generate weights0 Though the analysis is more complex, therefore,
Hillman's overall objective is the same as that of the other projects
mentioned, namely to identify semantic units0 Hillman 1969 reports
work on 1000 document texts, and Hillman 1973 describes an operational
retrieval service with substantial date files, but there is unfortunately
no indication either of the accuracy of the analysis procedure or of thevalue of its output•
The Syntol project (Syntol 1964, 1965, 1967, 1970) differs inthat the relational information extracted in analysis is carried over
to indexingo
It has made the most consistent and ambitious attack on
syntactic analysis and indexing, designed to generate document
descriptions similar to Farradane°s, and the attempt to produce such
sophisticated descriptions automatically is clearly of considerable
interest
The Syntol research is described in some detail in Sparck Jones
1973a* It is based on the assumption that the syntactic analysis
procedures required to identify the logical relationships to be
indicated in document descriptions must be based on relatively detailed
rules and dictionary information0
Initial attempts to work with arather crude analysis procedure using clue words, described in Syntol
1967, were not discriminating enougho However the syntax rules usedsubsequently were not derived from any systematic view of grammar, butwere developed ad hoc for French, and may well have been influenced by:he particular subject fields psycho-physiology, in which the system302

has been tested0 Though the analysis procedure is not required
to provide a full structural picture of sentences, it has to be
carried far enough to pick up key concepts and their relations,and this implies fairly detailed processinga
The experiments reported in Syntol 1970 were carried out on 1016
abstractso They involved a dictionary of about 7000 words or word
groups with detailed morphological and syntactic information, thelatter invoking 57 syntactic categories^ for example* The initial
function of the syntactic analysis routines is to identify word
groups and to resolve ambiguities open to syntactic treatment0 (Thereis concurrent semantic processing to select lexical items by reference
to a descriptor dictionary of some 3000 itemso) The tests showed agenerally satisfactory recognition of groups, and over 90% successin ambiguity resolution0 The second stage of syntactic analysis isdesigned to identify the syntactic function of specific words or groups,
and the type and structure of the phrases„ For example, the input
sentence "Modification du comportement du chien apres ablation dulobes orbitaux et frontaux" is divided into an initial subject phrase,
three prepositional phrases, and a final coordinate phrase0 Some 750
rules for exploiting the appropriate dictionary information are
required to carry out the analysis0 Inspection of a sample of 155
abstracts showed few absolute failures of analysis, though some phrases
received several interpretationse However the success of the procedure
is really to be judged by the descriptions generated, to be consideredbelow0III„lo2

Statistical syntax

This can be described as back-door partial parsing„ There is noattempt to characterise syntactic structures occurring in the input
text in the usual wayQ The assumption is simply that if two content
words tend to cooccur within a restricted context, like a sentence,
this must reflect the presence of a syntactic or logical relation
between them0 Pairs or n-tuples of words thus cooccurring may therefore
be selected to function as complex descriptors• The main difficultyis picking up significant frequencies of cooccurrence, since it isquite a strong requirement that two words should cooccur within asentence or other frame sufficiently frequently within a text for
their cooccurrence to be significant«
Early work on the Smart project (Salton 1968a,c) attempted to
identify "statistical phrases" intended for use as complex index
termso The process was not genuinely statistical, since reference
was made to a dictionary of acceptable word, or rather thesaurus
class0 combinations, cooccurrence of members of the relevant classes
in a sentence being the criterion for the assignment of the corresponding descriptoro Retrieval tests with three small collections
showed the procedure was not especially helpful, performance being
the same as that of a simple thesaurus. Salton attributes this to therestrictive prior dictionary rather than false combinations0

3.3

A rather similar approach was tried by Artandi 1969a,b, for the
automatic generation of links. It was restricted to relating wordsof two types9 drug names and °modifiers8 like "effect", defined byan indexing vocabularyQ Tests with 15 document texts showed 63 outof 285 incorrect links0 These were generally associated with greaterinter-word distancesp but Artandi concludes that simple distancecriteria would probably be too crude for full texts, where stylevariesp though they might be applicable to abstracts0 These experiments
were on too small a scale to be particularly informative.

Ill do 3

Full parsing

Full parsing for document description appears to have been
seriously attempted only in the early stages of the Smart projecto
The procedure is fully described in Salton 1968a0 Its object was to
identify phrases, like "retrieval of information", which could leadto the assignment of complex descriptors0 The Harvard Predictive
Analyser (Kuno 1965, 1966) was used to give a phrase structure parsingof sentenceso Specific substructures, for example those representing
a subject-verb relationship, were then matched against a dictionary of^criterion phrases0o A criterion phrase represents an acceptablecombination of wordsp or thesaurus classes, with syntactic dependency
relations between themQ Each phrase characterises a type of structure
which may be instantiated in a variety of specific ways in text* For
example a single noun+qualifier criterion phrase with the qualifier
dependent on the noun,? like "house+brick* could occur in actual text
as either "brick house" or "house of brick". The dictionary comparison
is not trivial0 since allowance has to be made for text and dictionary
structures which match in essentials but not in every detail.
At the time, experiments with the Analyser were relatively expensive
and only a limited number were conducted. The results of actual
retrieval experiments exploiting the technique were also not very
promisings with one small collection it turned out that these syntacticphrases performed less well than the statistical phrases describedearlier (and hence than a simple thesaurus)„ Salton's view is thatthe inadequacy of the grammar is probably responsible, and reliance ona prior dictionary may also account for some failures.

Illola4

Conclusion on input syntax analysis

Three points should be made0 The first is that even the mostambitious approach, the Syntol one, is orientated to some selectionof information from the input text. In question answering, in contrast,
all the information in the input text may be extracted for future use
in retrievalo The second is that only a few of the projects aredesigned to extract relational information which will actually be
explicitlv indicated in document descriptions (as opposed to implicitly
in compound descriptors)% the Syntol workers and Artandi represent
two extremes here. The third is that all parsers used are,3,4

by linguist's standards, inferior, since they are all of the phrase
structure typeQ Special problems from the documentalists' point ofview are presented by analysis procedures like the Syntol one, orSalton's, which require an elaborate word dictionaryc The effort
involved in constructing such a dictionary is substantial. The ideaof distinguishing closed and open word classes, and of using a limited
dictionary confined to closed class words like prepositions andconjunctionsP and perhaps suffixes, is an attractive one. The hypothesisis that information about these words can be exploited when they occur
in sentences to assign syntactic categories to other words, and hence
permit parsing,, The practical advantages of reducing the parsing
dictionary in this way are clear, The idea has been exploited forfull parsingp for example by Thorne 1968; it appears to be particularly
well suited to documentation where only partial parsing may be required.
In fact Hillman 1968 adopts this approach, and it is also followed by
Earl and Rush in their work on extracting.
In general >, it will be evident that it is extremely difficult tocomment on the effectiveness of the analysis procedures described:
evaluation really depends on the value of either the selected
descriptors or the identified relationships for retrieval*

III o 2

Description syntax

As noted9 the objective of input syntactic analysis is often
simply to identify key items for use as descriptors in indexing0 Thereis no intention of providing syntactically structured descriptions except
implicitly, where complex descriptors are usedc Salton treats his
phrases in this wayp for example• In general in indexing the treatmentof compound descriptors, or precoordinate subject specifications, may be
more or less refineds a string of words or terms may be regarded simply
as an indissoluble whole, or some distinction may be made between mainand subordinate elements, The latter, to be found in printed indexes,
is of course a move towards explicit syntax9 It is difficult to getmuch idea of the complexity of strings extracted automatically: Salton
1968a mentions phrases like "computer control"5 an example in Hillman
1973 includes both "rock bolts" and "roof support in undergroundexcavations30 0
and the output given in Klingbiel 1973b contains"interactive retrieval" and "automated indexing techniques".
The main project which has sought to express syntactic functions
and relations explicitly in document descriptions is the Syntol one(1964, 1965, 1967, 1970)„ A Syntol description consists of one or more
°syntagms9s these are pairs of index terms linked by general relations
like cause or association• The Syntol workers do not advocate any
particular relations: a Syntol-type language might use anything fromtwo to twenty broad logical relations. The tests reported in Syntol 1970
used three, the consecutive, comparative and associative relations. The
last is the least restrictive and is used as a sort of residual relation
if neither, of the first two holds. The detailed procedures forcreating syntagms for the first two relations thus differ from thosefor the thirdo

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The second major component of the Syntol document processing
package is therefore the procedure for translating semantic andsyntactic information associated with the phrases identified by the
analyser into syntagms. As indicated earlier, individual words are
replaced by index terms taken from a substantial hierarchicallystructured thesaurus0 Extraction of the first two relations involves
the use of clue words, like prepositions and verbs of certain sorts,and syntactic schemas specifying characteristic constructions
associated with the clues„ If a clue and schema match an analysedsyntactic structure, the appropriate logical relation between
indicated terms is establishedo Semantic checking involving aff
reseau notionnel1 which characterises collocational relations
holding between thesaurus descriptors may be required0 Associative
relations are normally supplied if consecution and comparison are
not establishedo The apparatus required is fairly complicated: forexample there are 30 schemas for the type of transitive verb clue
represented by Mprovoquer"0
The output from the procedure can be illustrated by an example.
The abstract text "Chez dix singes stimules par 4lectrochoc corticaldirect et chez un malade la procaine intraveineuse a protege del9epilepsie pendant une demi-heure? la xylocaine a un effet moindrew
gave syntagms depending on associative (A) and consecutive (C)relations as follows: PROCAINE C CRISE, PROCAINE C EPILEPSIE, PROCAINEA PROTECTION, PROTECTION A EPILIPSIE, DIRECT A CORTEX, ELECTROCHOC A
STIMULATION, STIMULATION A ETRE, XYLOCAINE A - 0 The test results
obtained were evaluated by comparison with manual indexing for asample of the abstracts processed0 The syntactic procedures forgenerating syntagms gave 97% correct results? but, surprisingly, thesemantic checking reduced this to 73%0 Some details of the evaluation
are unclear? and it is worth noticing that nearly 20% of the indexing
expressions consisted of isolated thesaurus descriptors, and that 80% of
the syntagms involved the very general associative relation0
At a lower level, Artandi 1969a,b generates index descriptions
involving links between controlled language terms« As noted, 22% of
the links found in 15 document texts were judged incorrect*

III o 3

Index language syntax

Recent general discussions of index language syntax are to befound in Gardin 1973 and Coates 1973„ Syntax appears in an indexing
language a) in the indexing vocabulary if this consists of compoundterms or subject headings, or b) in the set of relations which may
be used to link termsQ For manual indexing there is a large literatureon the construction of subject vocabularies, and on techniques for
explicitly indicating relationships, for example by the UDC • :',facet order, relational operators (Farradane 1967, 1973), etc. I shall
ignore this here0

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Fully automatic text derived syntactic indexing has not beenmuch attemptedo Hillman's 1968, 1969, 1973 approach is derivative,
but most of the approaches mentioned rely on the control imposed bya dictionary, either indirectly through word or term selection, or
directly as in Salton°s 1968a criterion phrase technique<,III » 4

Search syntax

Machine searching of manually assigned subject headings is notin question here0
With compound descriptors, any procedures which select a component,say by exploiting a hierarchical relationship to replace XYZ by Z,might be claimed to be syntactic, but it is perhaps more appropriate
to regard them as semantic *When syntactic information is explicitly provided in document
descriptions, obvious ways of modifying it during searching are by
simplifying expressions and by weakening relations0 For example adocument characterised by the expression CA rel B rel C 8 may beallowed to match a request consisting of 'A rel C°; alternatively,
if we have a set of specific relations, relj, rel2 and rel3 withrel3 weaker than the others, we may allow a match on rel3 as a
substitute for one on the others0 In principle permitted operations
should be defined by rule and not specifically by the provision ofalternative forms of a request0 Of course manually generated descriptionsof this kind could be manipulated automatically in searching, as Farradane
has suggestedp but it does not seem to have been attempted, other thanin an early stage of the Syntol work (Syntol 1964): the results ofthese experiments suggested that relations must be weakened to retrieve
sufficient relevant documents0 In the recent research on automatic Syntol
indexing (Syntol 1970) there have been no actual retrieval experiments„
Clearly, if syntactic information is incorporated in document
descriptions, the treatment of requests must correspond,, Automatic
request processing is illustrated by Hillman's LEADER system forinteractive searching (1968, 1969, 1973) and by the Smart project(Salton 1968a) c However if documents are indexed by straightforward
keyword strings, these may be extracted manually from requests*
As noted earlier, Boolean requests can be said to have a syntactic
structure which may not be explicit in document descriptions0 Boolean
requests are ordinarily prepared manually, and are very common in
operational systems, for example UKCIS (Barker 1972a,b), IITRI (Williams
1972)
A good deal of effort may be involved, and care, for instancein the treatment of not-logic (Scheffler 1972) 9
These remarks apply to the processing of submitted requests or
profiles* Their original generation is a somewhat different matter*,
Carroll°s 1973 experiments with parsing are of interest since they
were designed to generate profiles from source documents0 Retrieval
comparisons between 15 machine and Boolean profiles for about 10000

3.7

documents showed improved recall with comparable precision for themachine profiles,. The technique involved is rather like request
modification by relevance feedback in iterative searching, to beconsidered laterc

III , 5Conclusion on syntactic indexing

The general value of syntactic information for retrieval has
been strenuously argued. In the present context there are several
distinct questions to examineu
We must first separate syntactic criteria from syntactic
descriptions9 that is procedures using syntactic information simply
to identify semantically important items to be used for describing
documents from procedures retaining syntactic information for its
own sake. In this case the question is whether document keys thus
identified are more valuable as descriptors than ones obtained without
reference to syntax, for instance statistically. A subsidiary questionis whether, assuming such keys are preferable, they can be adequately
recognised automatically. In fact in manual indexing syntactic criteria
may well be exploited, unconsciously, as a natural aid in reading text*
But there is no obvious way of conducting a sensible experiment in thecomparative automatic and manual use of syntactic criteria for
selecting wordsQ It is more useful to ask whether syntactically
based automatic extraction methods are preferable to non-syntactic
ones0 Unfortunately there is rather little evidence to go on* Of
the work described, such projects as Borkowski's, for which good
performance is claimed, are too specialised to be relevant0 Hillman
does not provide any concrete performance figures for his system, or
attempt to evaluate it other than sociologically, and Klingbiel has
not yet attempted searching0 Salton does provide some limited
comparative results* As mentioned earlier, performance for his
syntactic phrase procedure, over 17 requests and some hundreds of
documents, is noticeably inferior to that of statistical phrases0
Inspection of other performance figures in Salton 1968a suggests it
is much like that of simple keyword stems taken from the texts*
In considering the value of syntactic information in descriptions,
we can first ask whether the implicit syntactic information of
precoordinate descriptors is of value, and secondly whether explicit
information is helpful, with a further distinction between relatively
sophisticated information like that provided by Syntol encodements
and simple linksc Again, if it can be shown that such information isof value,, we can further ask whether it can be effectively picked up
automaticallyg
It is not appropriate here to rehearse all the arguments for andagainst precoordinate descriptors, for the use of minimal syntactic
links, or for more ambitious indexing of the type advocated by
Farradane and implemented, for instance, in the Titus system (Bousselet1973)o The following points are perhaps sufficient, Evaluation
experiments designed to compare, for manual indexing, precoordinate

3o8

subject indexing with post coordinate terms, or syntacticallystructured descriptions with simple term lists, suggest that theuse of either implicit or explicit syntactic information in document
descriptions is of no general material value. See, for example,
Saracevic 1968, 1971, Cleverdon 1966, and Keen 1972, 1973. At thesame time, pcstcoordinate subject indexes continue to be made, andsystems like the Titus one exist. It must be allowed that syntactic
descriptions may be of value in specific contexts, but there is no
hard evidence to the effect that they are generally of value.
This being the case, the justification for automatic syntactic
indexing appears dubious. The few evaluation experiments which havebeen conducted, like those of Salton's just mentioned, suggest that
automatic syntactic indexing is neither better nor worse than its
manual parallel. However, it may be that the correct way ofproviding syntactic information in document descriptions has not
yet been discovered, and that when it has, it may be worth writing,or attempting to write, programs to provide it automatically.

3.9