72

Online public accesscatalogues

The phrase Online Public Access Catalogue (OPAC) is used in this report
to mean an online catalogue which is the main or only catalogue for theusers of a library. This excludes from the discussion those computerised
circulation control and cataloguing systems which can be used forcatalogue search but are normally so used only by library staff; somelibraries have made terminals to such systems available to those generalusers who care to learn how to use them — these systems are not counted
as OPACs unless they are the chief means of catalogue access.
This chapter is not meant to be a comprehensive introduction to thesubject. The topics which have been covered are those which have somebearing on this research and on the design of Okapi, the project's
prototype OPAC.
2.1

Sources of information on OPACs

The team learnt about OPACs by looking at them, by reading and bytalking to OPAC users, designers and researchers.
2.1.1

Existing OPACs

There are still few OPACs in this country and a majority of these are Geac
circulation control systems, some of which have certain enhancements,
often done locally with assistance from Geac — the University of Hullsystem allows Boolean searching, the University of Sussex has an
approach to rotation of title keys. The team observed the use, and talked
to staff and users, of the Geac catalogues at the Polytechnic of the SouthBank and the London Borough of Bexley. In July 1984 two of the teamspent a day with the University of Cambridge's interesting andunconventional OPAC [1], which was developed by library automation
staff there on a very low budget. During a visit to the United States in1983 Mel Collier looked at the Pikes Peak District Library system and the
University of California's MELVYL (see below).

2. Online public access catalogues

8
2.1.2

Written sources

GENERAL

There are two articles by Alan Seal in [2] which form a clear and very
concise introduction to the subject, but the single essential written source
is Charles Hildreth's excellent monograph [3]. The coverage of
Hildreth's book is much wider than the title suggests, and after four
years, which is a long time given the pace at which technology is moving,
it is still almost all pertinent. There is a "guide for managers" byMatthews [4], which includes brief descriptions of nearly forty systems;
it is useful for reference in spite of containing many typographical errors.
Fayen's book [5] also contains some useful material, but the subject is,perhaps, treated from the librarian's point of view rather than from thatof users. Markey's book [6] on subject access is extremely valuable,
although it does call for some prior knowledge of OPACs and OP AC
research. This book is mentioned below in connection with the use andevaluation of OPACs.
SPECIFIC OPACS

The fullest readily available source of information on a specific system isthe excellent series of articles on MELVYL published in Information
Technology and Libraries [7,8]. These describe the design, development
and use of this elaborate and ambitious OPAC in considerable detail andwithout glossing over the difficulties encountered or the shortcomings ofthe resulting system. In some ways, the most ambitious and advanced
existing OPAC is the National Library of Medicine's CITE, developed
by Doszkocs and others. Published material on CITE [9,10,11] is not as
informative as it might be, but is nevertheless essential reading fordesigners of future systems. (CITE is briefly described below in Section
2.4.4.)CATALOGUE RESEARCH

Since most users have no other model to apply, it is reasonable to assume
that many of them approach OPACs with something of the preconceptions which they have about conventional catalogues. Hence studies of
the use of conventional catalogues are highly relevant. Of particularimportance are the group of papers by Tagliacozzo, Kochen and others[12,13,14] and the work of Krikelas [15]. There is a list of catalogue use
studies and a discussion of their methods in Chapter 2 of Markey [6].

2. Online public access catalogues

9On the evaluation and use of OPACs there was an extensive research
project which was carried out for the Council for Library Resources(CLR) in the United States, and which resulted in a a number ofpublications from the various participating bodies, including a three
volume "Final report" from the Online Computer Library Center(OCLC) [16,17,18]. This was the outcome of a large amount of planningand work, but the concern in the report with statistical generalisations
can be misleading if quoted after only a cursory reading. Much valuable
material underlies the report and this is available for further analysis andresearch. It includes transaction log tapes and interview and questionnaire data.
Karen Markey, the author of volume II of the final report of the CLR
study [17], expands this material, to considerable effect, in her book on
subject access [6]. Although we did not see Markey's book until ourdesign ideas had already hardened, it should be required reading for
future designers.
OTHER MATERIAL

As well as the above sources, there are two or three hundred relevant
journal articles and occasional papers, many of which one or more of the
Okapi team has read. These are to be found in the librarianship literature,
in sources concerned with information science and computing, and in thefields of human-machine interaction and human factors or ergonomics. A
bibliography of some two hundred items has been compiled, broadlyclassified under nine facets. This may be published as a separate
annotated bibliography.
2.2

OPACs as IR systems

2.2.1

Coverage and source

One of the differences between OPACs and conventional IR systems isthat online catalogues often access references to bibliographic material inmany subject areas, if not the whole of knowledge (as well as fictional
material). ''Traditional" online or in-house reference retrieval systems
usually cover one or a few narrower fields in which terms are less contextdependent, so that it is feasible to build a thesaurus.

2. Online public access catalogues

10

Another important aspect of OP AC files is that their content is usuallylimited to what is in MARC records: few libraries have been prepared to
key in catalogue records since a large proportion of these are available
from cooperatives in MARC format. These records do not have abstracts,
assigned (controlled) indexing is often poor or too broad — Library ofCongress Subject Headings for example — but there is some classification
scheme, most commonly Dewey or Library of Congress.
2.2.2

Users and usability

The major requirement of an OPAC is that, unlike a conventional online
reference retrieval system, it must be usable by people with widely varied
backgrounds, without an intermediary: library users cannot be expected
to know much about computers, catalogues or IR techniques, nor is it
reasonable to expect them to learn them. The only suitable way in whichusers can be assisted is while they are actually using the OPAC, that is by
the system itself (help messages, suggestive prompts, error messages),and by instructional material in the form of leaflets or brief instruction
sheets on display near the terminals.
Because OPACs must be readily, efficiently and, if possible, pleasantly
usable by at least as broad a section of the community as that which uses
conventional catalogues, and preferably by all those who use libraries forany purpose, it can be said that they must combine features of IR systems,
cash dispensing machines and computer games (Section 7.3). The userinterface design is absolutely crucial.
To summarise, an OPAC can be characterised as being an IR system (a)for users from widely varied backgrounds, (b) which needs no intermediary, (c) whose records are not restricted to a single (or a few) subject
areas, and (d) its records are usually MARC or MARC-derived, enriched
only by controlled subject descriptors (subject headings and classification
numbers) which are often too specific or too general.
2.3

From conventional catalogues to OPACs

Participants at a conference in 1980 were asked to give a definition of an
online catalogue. Some of these were lists of desirable features rather thandefinitions, but two of them are quite succinct (quoted by Fayen in [5]):2. Online public access catalogues

11"[An online catalog provides] online access to the complete
bibliographic record of all the Library's holdings with minimal
access points being the same as those in a card catalog." (National
Library of Medicine.)"A true 'On-line Public Catalog' should provide sophisticated accessfor unsophisticated patrons. It should remove from the patron theburden of any interpretation of library procedures and practices".
(Pennsylvania State University.)
It is clear that OPACs have certain inherent advantages over conventional
catalogues. The principal advantage is that they can easily be made toprovide a much greater range of access points. Another advantage, which
until recently obtained mainly in the more primitive OPACs derived
from circulation systems, is the inclusion of availability information andusers' ability to make reservations. Most writers would assume that all
OPACs in lending libraries must provide this function [2].
It is less immediately clear, but must be appreciated, that OPACs also
have a number of serious disadvantages. Perhaps the most important ofthese is that ease of use tends to vary inversely with the sophistication of
the functions provided. This has resulted on the one hand in OPACs
which are easy to use but ineffective, and on the other in OPACs offeringenhanced access and sophisticated facilities which are not taken
advantage of by most of their users. This has been cogently but perhaps
over-pessimistically expressed by Lipow: " 'user-friendly' to me issynonymous with limited service" [19]. It is generally true that even thesimplest OPAC gives the user a feeling of being more highly involved
than do most conventional catalogues. The almost unavoidable use of a
keyboard for user input is also an important source of user frustration andof searches which fail because of spelling and keying mistakes.
2.4

Types of OPAC currently in use

There are probably now (early 1985) between fifty and a hundred distinct
OPAC systems in use. Most of these are in the USA, with a handful in the
UK and the rest of Europe. Some are commercially available,
increasingly now as part of integrated library systems. Many weredeveloped as in-house systems by academic, national and public libraries.
It may be helpful to regard OPACs as falling into three categories, ofwhich the first is, in the main, historically the oldest. These are roughly2. Online public access catalogues

12

equivalent to Hildreth's "three generations" of OP AC which he definedin a recent paper [20], but it should be realised that it is not always
possible to place a given OP AC firmly in a specific generation.
The first and second generations are most readily distinguishable by thetypes of access key provided.
2.4.1

First generation OPACs

OPACs of the first generation will be referred to in this report as phrase
indexed or pre-coordinate OPACs. Their access points are usually similar to
those of a hard copy catalogue: author, title (as a phrase), class mark orcall number, sometimes subject heading(s) — as a phrase. There may also
be "derived" or "acronym" keys which the user has to know how toconstruct. It is a characteristic of this category that to find a title, or a
subject heading, a user must enter either a phrase (at least the beginningof it), or else a derived key.
Of OPACs in this category, some are derived from circulation control
systems, some from cataloguing systems and some were designed as
OPACs. A pioneering example is the LCS (Library Control System)
OPAC from Ohio University, which has been operational as an OPAC atleast since 1978, and since 1970 for circulation control [4]. Other
examples include the Pikes Peak Library District system [4] and the many
Geac circulation control systems which are being used as OPACs in thiscountry.
One of the features of this type of OPAC is that a search usually results inthe display of something: an author mistyped as SMTH might result in adisplay of a number of brief records including
SMOUT T C. A history of the Scottish people.
SMULLYAN R M. What is the name of this book?
as the two "nearest" author entries to SMTH, and it is normally possible
to browse backwards and forwards in alphabetical sequence. The usermight find SMITH by browsing backwards through this author
sequence, without even noticing that s/he has mistyped it. This can be auseful feature. The process of searching is not very dissimilar fromsearching a conventional catalogue, with the disadvantage that entries

2. Online public access catalogues

13

have to be keyed as well as recognised, rather than just recognised as they
are when a search is done by scanning cards or microform/ 0
2.4.2

Second generation OPACs

These are referred to as keyword or post-coordinate OPACs. Their accesspoints are similar to those provided by the traditional online reference
retrieval systems such as Lockheed DIALOG: words from the free andcontrolled text of the bibliographic records.
These OPACs differ from the reference retrieval systems in the nature of
their records and in the mode of user interaction. The OP AC records,
being usually MARC or MARC-derived, do not have abstracts, nor dothey have controlled indexing except, sometimes, for subject headings;
thus access points are words from title-like fields, author and other names^
and words from subject headings when these are present. As regards userinteraction, it was obvious to most OP AC designers(2) that, since online
reference retrieval systems need trained intermediaries to mediate
between the user and the system, users would not be able or prepared totackle the task of learning a fairly complicated command language. The
approach taken in keyword OPACs has often been to try to provide amuch simpler (and more limited) approach to user interaction.
Several second generation OPACs have two levels of user interaction — asimple one for ''naive users", and an advanced one which may be the fullcommand language of a typical Boolean IR system. At the simple level,normally only the Boolean AND is available: in a title or subject search,
the user can enter a phrase, and the system will assume an AND between
the individual words.
In many cases, OPACs of this type have their own software, but several
make at least partial use of existing IR system sofware. Examples are theUniversity of California's MELVYL (own software for access with file
management by ADABAS) [7,8] and the Dartmouth College system(modified BRS software) [22].
(1) A VDU screen is doubtless quicker to scan than a deck of cards, but the amount of
information which can be carried on a screen is far lower, and often less readable, thanthat in a microfiche frame. Hence it is easy to understand the appeal which these types of
OPAC had in the United States, where few libraries introduced microform catalogues,
but rather went straight from card catalogue to OPAC. The reasons for their appeal to
UK librarians are less obvious.
(2) A few libraries have assumed that users may be able and willing to learn a commandlanguage — see for example the OPACs at the Library of Congress referred to by Arretin her paper on learning to use OPACs [21].

2. Online public access catalogues

14

The appeal of keyword systems is largely because they provide a muchlarger number of access points than do pre-coordinate systems, together
with the well-developed state of Boolean IR system design. Obviously
searches of an OP AC which processes title or subject phrases by implicitly
ANDing the constituent words are not affected by misapprehensions
about word order; on the other hand post-coordinate searching of large
files results in a high proportion of "false drops".
In contrast to the first generation systems, a search of a keyword system
retrieves a set of records, and sets may be empty, or unmanageably large.
Although it could be said that users can "browse" through the records ina retrieved set, this is very different from the way items are located bybrowsing in a phrase-indexed OP AC (and one cannot browse an empty
set).
Some of the more recent OPACs, particularly those now being offered aspart of an integrated library system, although basically keyword, postcoordinate systems, do also allow phrase indexing and searching [23,24].
It will appear later (Section 2.5) that it is fairly obvious that both types ofaccess are necessary in an OP AC.
2.4.3

The third generation

Reverting to the Pennsylvania State University OPAC definition quoted
at the beginning of Section 2.3, this would be satisfied by a third
generation system, but it is doubtful whether any such OPAC yet exists.
According to Hildreth [20, p41], they would feature both phrase andkeyword access, their bibliographic records would be enriched by the
inclusion of additional controlled and uncontrolled access points, theywould accept search expressions in "ordinary language", provide context
dependent automatic help and guidance and a degree of error-correction— in short, they would combine some of the functions of a reference
librarian with those of a catalogue.
Several OPACs have certain third generation features, most notably the
National Library of Medicine's CITE system [9,10,11], and to someextent also the Beth Israel Hospital system, Paperchase [25], and thisproject's Okapi. In addition, as mentioned above, there are now somesecond generation OPACs which offer both phrase and keyword access.
All the third generation features listed by Hildreth could be implemented
to some degree, using techniques which are currently available in IR and2. Online public access catalogues

15in the design of interactive computer systems. This is not to say that weknow even approximately how to implement them satisfactorily —merely that the techniques are available, and what is needed is a major
research effort to learn how to apply them. Some of the more important
aspects of research and development are discussed in more detail in
Section 9.4.
2.4.4

The National Library of Medicine (NLM)

CITE OP AC

Since CITE will be referred to quite frequently in this report, a very brief
description may not be out of place here. CITE is the catalogue for a fairly
large collection (at least 500,000 items) of medical monographs. It uses a
subject searching technique which has been little tried elsewhere except
in experimental IR systems/ 0
The user enters a search statement in "natural language", rather as it
might be presented to an online search intermediary — for example"Kidney disease in infants and newborns". CITE then performs a searchusing the words from the search statement together with morphologicallysimilar terms and related subject headings (MeSH headings). (Thecombinatorial search technique used in Okapi is substantially the same asthat of CITE, and is described in Section 6.5.) This will result in thedisplay of a number of catalogue records, some of which may be relevant.
The user is then required to indicate which records are relevant andwhich terms from them are important, the search is reprocessed, andthese steps are repeated until the user decides to stop.
Some of the techniques used in CITE cannot be applied to general
catalogues or their users. The ways of obtaining related terms and subject
descriptors are subject-specific and dependent on the existence of a set ofwell-defined subject headings. The users of the N L M are medical
professionals and students, and cannot be regarded as being similar tousers of general academic or public libraries: users need a rather highdegree of motivation, as well as subject knowledge, to obtain satisfactory(1) CITE was also one of the subjects of perhaps the only comparative evaluation of two
OPACs under operational conditions in the same library [26]. It was tested against themore conventional ILS OPAC (since enhanced, and adopted by OCLC for its LS/2000"local system"), and found to be more effective and generally preferred by users of the
medical library.

2. Online public access catalogues

16

results. Nevertheless, it would certainly be possible to implement systemsof this type with other moderately specialised collections, and the successof CITE does suggest some of the lines along which OPAC research anddevelopment should proceed.
2.5

Types of catalogue search

It has been widely accepted that user searches of library catalogues fall
mainly into two categories, specific item and subject searches. It is better to
use the expression specific item rather than the more frequent known item
because quite a high proportion of searchers are looking for a specific item
but have only an imprecise specification for it (in extreme cases a specificitem searcher may have neither the author nor the title, but only theknowledge that such an item exists and s/he would recognise it on sight).
Several other types of search have been identified, including bibliographic
searches (checking references) and author searches (checking holdings ofworks by a specific author) [27].
The proportion of subject searches decreases with the academic level ofthe user, and is high in public libraries. The overall proportion of subject
searches shown by various studies of conventional catalogues ranges
between 62% and 10%, while the figures from OPAC use studies lie
between 65% and 34% (the lowest figure was obtained in a study of acatalogue which offers subject access only by pre-coordinated LCSH)[6, p76].
Certainly the proportion of subject searches is higher in OPACs than it isin conventional catalogues.
Studies such as that of Tagliacozzo and Kochen [12] on the use of card
catalogues indicated that most searches were of the author/title catalogue
but that a substantial proportion of these were "hidden" subject searches.
In some cases the reverse happens: subject catalogue searches areattempts to locate imprecisely known specific items.
Most conventional catalogues encourage and perhaps even give rise tothis specific item/subject dichotomy because there is an author/title

2. Online public access catalogues

17

sequence and a subject or classified sequence (except in some dictionary
catalogues).
2.5.1

Specific item searching

Most searchers arrive with an author and/or title rather accurately
specified. Although they may be misspelt, the most significant parts (lastname of an author, first few words of title) are usually substantiallycorrect [13], with titles more likely to be correct than authors. In a
conventional catalogue this means that most specific item searches can besatisfied by browsing an author/title sequence in the region of the entry
point. Similar browsing can be done with the pre-coordinate, phraseindexed type of OPAC, but not with a purely keyword system. Hence,
keyword approaches do not seem particularly well suited for specific item
searching: a majority of searches succeed, if they are going to succeed atall, on a phrase search. However, a keyword system can work fairly
satisfactorily with titles; it does of course succeed when there are wordtranspositions, but spelling mistakes in title citations are far more
frequent than mistakes in word order.
2.5.2

Subject searching

It is generally felt (for example by Markey [6]) that it is in the area ofsubject access to library materials that OPACs can show their realadvantages.
Here there is a divergence between US and UK catalogue practice. Most
US libraries do not make use of classification for subject access, relying
rather on LCSH or other subject terminology consisting of precoordinated headings and subheadings. One of the major difficulties insubject access has been users' inability to find or guess the exact subject
heading, or at least its beginning. Hence there has been much emphasison the potential of keyword-type OPACs for giving access to thecomponent words of subject headings, thus making them available asterms to be used in post-coordinate Boolean-type searching.
In the UK, on the other hand, subject access to conventional catalogues
has usually been via a classified sequence accessed by means of a printed
index relating subjects to classification numbers. Although this is a twostage process it may be more effective than many libraries' use of LCSH,
because related subjects are more likely to be adjacent in a classification
schedule than they are to be alphabetically adjacent in a subject heading2. Online public access catalogues

18

list. Many MARC records in the UK do not contain subject headings, so
in many OPACs the user would have to consult the printed subject index
and then search by class number, just as in a conventional catalogue. The
Geac system at the Polytechnic of the South Bank has a subject index
online [28], thus avoiding the necessity of searching the printed subject
index.
Many MARC fields other than subject headings and classification
numbers can contain subject-rich information, most notably titles andsubtitles, but also series titles, corporate names, and some of the notes
fields (when these are used). In Appendix F of [6], Markey gives a list,
which is comprehensive apart from the omission of cross-reference fields,of subject-rich fields and subfields of MARC records.
The most obvious and simplest approach for subject searching is to use
post-coordination of keywords. This is often implemented by allowing
access by any word or words from title fields and subject headings, if
present, and assuming Boolean ANDs between words. Certainly an
online search intermediary would often obtain better recall and precision,
but this simple method may be at least as effective as forcing the user totry to find the correct subject heading(s) or class number(s).
That a purely post-coordinate approach is not ideal for subject searching
was demonstrated by examining the results of a search for items on
library science in a keyword system. With the Okapi prototype searching
the PCL catalogue this gave a precision of about 30% on 216 items
retrieved.
2.6

User interaction with OPACs

This material is covered in Sections 7.2, 7.3 and 7.4. It is introduced here
because of the prime importance of user interaction in OP AC design.
2.6.1

User attitudes and behaviour

Surveys show that most users of most OPACs seem to like them. The
CLR survey also found a generally favourable attitude among non-users.
The first experience of the catalogue is important in determining a user's
attitude. It is felt that users' expectations will increase and that attitudes
may, and should, become more critical as people have more exposure todifferent OPACs [18].

2. Online public access catalogues

19

Features which need an appreciable effort to learn or use tend to be little
used. Where different levels of user interaction, "expert" and "novice",have been provided, most searchers choose the easier level, despite thefact that this may deny them access to the more powerful facilities of thecatalogue [29].
Usage of a feature is highly dependent on its ease of access. The
University of Sussex wanted its combined author/title derived key search
to be used, because it is effective and does not need much processing
power; merely moving it to the top of their menu increased its usage
dramatically [30]. This suggests (what is borne out by studies of othertypes of interactive systems) that users tend to take the first option whichappears at all satisfactory, and it follows that only a small number ofcourses of action should be offered at one time.
There is also a tendency for users to assume that they "know" aninteractive system after they have had a few successful experiences with
it; thereafter their level of expertise does not increase, so that they may beunable to carry out tasks which are within the capability of the system butwhich did not form part of their initial experience. Most OPACs offer
online help facilities, but it has been found that online help screens whichhave to be explicitly requested by the user are little used [6]. This is the
motivation behind Hildreth's "third generation" requirement that
OPACs should offer automatic help and guidance wherever possible.
However, this must be done gracefully and unobtrusively.
2.6.2

OPAC input from users

Almost all systems require users to type not only commands or choices(menu selections) but also their search statements. Surveys show thenumber and distribution of "errors" made by users of systems which
expect specific commands, but they do not show the extent and effect ofspelling mistakes and miskeyings in search terms. It is to be expected that,particularly with keyword systems, misspellings will be a common cause
of search failure. A further very important factor, which has scarcely been
mentioned in the literature, is that most people type very slowly and makemany corrections. It would seem likely that for most users who arrivewith reasonably accurately specified known item requirements, microform catalogues would be quicker than OPACs (although an OP AC may
be quicker than a large card catalogue).

2. Online public access catalogues

20

Whether OPAC searches tend to be slower than searches on conventional
catalogues, or whether it is because people find them more fun or more
effective and so search more, it seems that the number of VDUs required
is greater than the number of microform readers needed to service thesame population.
The only alternative to keyboard input in OPACs has been the use of
touch-sensitive screens. It seems that they might have a place when madeavailable as an alternative to a keyboard. They are evidently not suitable
for the input of keywords to a post-coordinate system.
2.6.3

OPAC output

Since output is usually to a conventional VDU with 24 lines of 80
character spaces, it is not possible to provide a browsing display ofanything other than brief one or two line records. Most OPACs provide atleast two levels of display for the public: brief and less brief. Either type ofrecord may include circulation information. Many OPACs also provide a"full" record, but these are mainly for use by staff. A series ofexperiments by the Centre for Catalogue Research showed convincinglythat almost all catalogue searches can be satisfied by records which arevery concise compared to the entries in traditional card or microform
catalogues [31].
The typographical quality of VDU displays is extremely poor incomparison with printed text. It is difficult to produce output which canbe read fast, comfortably and accurately.
OPAC surveys have shown that many users would like printed output onrequest, and this is provided by a few systems, which have printers
attached to some of the terminals.
2.7

OPAC design

2.7.1

OPAC components

To a large extent there is only a loose connection between the searchfunctions of an OPAC and the modes by which these functions areaccessed: one can build two IR systems which look very different on thesame functional substructure. This is well exemplified by the "elementary" and "advanced" versions of MELVYL or the Dartmouth College
system; other examples are the many experiments on giving untrained2. Online public access catalogues

21

users access to dial-up online reference retrieval systems. In addition,
there is of course the question of the content, structure, storage andmaintenance of the bibliographic and other files accessed by the OPAC.
Thus, an OPAC can be regarded as having three components: its data
structures, its functional aspects, and the modes of user interaction.
In planning the design of an OPAC each of the components influences
both the others. As in any other elaborate design process, constraints,
either chosen, or else imposed by financial, hardware or otherconsiderations, can simplify matters by removing many of the degrees of
freedom. There is some truth in the statement that first generation
OPACs were mainly constrained by file structures and second generation
by functional considerations. To make a third generation OPAC it will benecessary to start with the user interaction component.
2.7.2

The design of Okapi

The initial constraints on the design of Okapi were those imposed byhardware and the necessity of using as source file the tapes used to
produce the Polytechnic's COM catalogue. However, having read
Hildreth [3], and having some experience of the use of interactive
computer systems by untrained users, the team was aware of the prime
importance of the interaction component.
The comparative slowness of disc access in the project's first LAN (theNestar Cluster/One — Section 3.4) meant that the functional component
had to be designed so that searches entailed an absolute minimum of discaccess, and it was by no means obvious that any but the simplest Boolean
combinations (ANDs on small sets) would be feasible. An index structureand search procedure which would only need a single disc access for mostsearches was evolved. The alternative to keyword post-coordination is toprovide a degree of pre-coordination by means of an index, at least ofphrases from title-like fields, containing entries which are informativeand readable enough to be usable as search results. (The display of index
entries is discussed in Section 5.3.)
The acquisition of the faster PLAN 4000 network allowed the disc accessconstraint to be relaxed. This historical accident may have been partly
responsible for the fact that Okapi emerged as a system combining preand post- coordination, and that so much attention was paid to indexing.
The more advanced hardware also allowed the incorporation of the2. Online public access catalogues

22

computationally quite demanding combinatorial search, and madefeasible the automatic search sequencing (''search t r e e s " ) . Both of thesefeatures are described in Chapter 6.
T h e r e is an account of the Okapi indexing in Chapter 5, and thefunctional aspects of design are discussed in Chapter 6. On userinteraction Chapter 7 deals in some detail with the topics mentioned in
Section 2.6 and gives a fairly full account of the rationale behind thedesign of the user interface.
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H i l d r e t h C R. Online public access catalogs: the user interface. O C L C ,1982.
M a t t h e w s J R. Public access to online catalogs: a planning guide formanagers. Online Inc, 1982.
F a y e n E G. The online catalog: improving public access to librarymaterials. Knowledge Industry Publications, Inc, 1983.
M a r k e y K. Subject searching in library catalogs: before and after the
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