Reprinted from Sciences, July 15, 1955, Vol. Citation Indexes for Science A New Dimension in Documentation through Association of Ideas “The uncritical citation of disputed data by a writer, whether it be deliberate or not, is a serious matter. Of course, knowingly propagandizing unsubstanti- ated claims is particularly abhorrent, but just as many naive students may be swayed by unfounded assertions pre- sented by a writer who is unaware of the criticisms. Buried in scholarly journals, critical notes are increasingly likely to be overlooked with the passage of time, while the studies to which they pertain, having been reported more widely, are apt to be rediscovered.” (7) In this paper I propose a bibliographic system for science literature that can eliminate the uncritical citation of frau- dulent, incomplete, or obsolete data by making it possible for the conscientious scholar to be aware of criticisms of earlier papers. It is too much to expect a research worker to spend an inordinate amount of time searching for the biblio- graphic descendants of antecedent pa- pers. It would not be excessive to de- mand that the thorough scholar chgck all papers that have cited or criticized such papers, if they could be located quickly. The citation index makes this check practicable. Even if there were no other use for a citation index than that of minimizing the citation of poor data, the index would be well worth the effort required to compile it. This paper considers the possible util- ity of a citation index that offers a new Eugene Garfield approach to subject control of the litera- ture of science. By virtue of its different construction, it tends to bring together material that would never be collated by the usual subject indexing. It is best de- scribed as an association-of-ideas index, and it gives the reader as much leeway as he requires. Suggestiveness through association-of-ideas is offered by conven- tional subject indexes but only within the limits of a particular subject heading. If one considers the book as the macro unit of thought and the periodical article the micro unit of thought, then the cita- tion index in some respects deals in the submicro or molecular unit of thought. It is here that most indexes are inade- quate, because the scientist is quite often concerned with a particular idea rather than with a complete concept. “Thought” indexes can be extremely useful if they are properly conceived and developed. _ In the literature-searching process, in- dexes play only a small, although signifi- cant, part. Those who seek comprehensive indexes to the literature of science fail to point out that such indexes, although they may be desirable, will provide only a better starting point than the one pro- vided in the selective indexes at present available. One of the basic difficulties is to build subject indexes that can antici- pate the infinite number of possible ap- proaches the scientist may require, Pro- ponents of classified indexes may suggest that classification is the solution to this problem, but this is by no means the 122, No. 3159, pages 108-111. case. Classified indexes are also depend- ent upon a subject analysis of individual articles and, at best, offer us better con- sistency of indexing rather than greater specificity or multiplicity in the subject approach, Similarly, terminology is im- portant, but even an ideal standardiza- tion of terminology and nomenclature will not solve the problem of subject analysis. What seems to be needed, then, in ad- dition to better and more comprehensive indexes, alphabetical and classified, are new types of bibliographic tools that can help to span the gap between the subject approach of those who create documents —that is, authors—and the subject ap- proach of the scientist who seeks infor- mation. Since 1873 the legal profession has been provided with an invaluable re- search tool known as Shepard’s Citations, published by Shepard’s Citations, Inc., Colorado Springs, Colo. (2). A citation index is published for court cases in the 48 states as well as for cases in Federal courts, Briefly, the Shepard citation sys- tem is a listing of individual American court cases, each case being followed by a complete history, written in a simple code, Under each case is given a record of the publications that have referred to the case, the other court decisions that have affected the case, and any other references that may be of value to the lawyer. This type of listing is particu- larly important to the lawyer, because, in law, much is based on precedent. Citation indexes depend on a simple system of coding entries, one that re- quires minimum space and facilitates the gathering together of a great volume of material. However, a code is not abso- lutely necessary if one chooses to compile a systematic listing of individual cases or reports, with a complete bibliographic history of each of them. Thus, it would be possible to list all pertinent references under each case with sufficient com- Mr. Garfield is Director of the Institute of Scientific Information, ]122 Spring Gar den Street, Philadelphia 23, Pa. pleteness to give the index more of the appearance of a bibliography. However, this would result in an extremely bulky volume. There are analogies in bibliographic operations, For example, in cataloging books for booksellers’ or library catalogs, an attempt is made to find references to each book in one or more adthoritative bibliographic sources, such as the’ cata- logs of the British Museum (BM), Biblio- théque Nationale (BN), or the Library of Congress (LC). The “authority” card used in cataloging sometimes looks like a Shepard entry. Another example is a book-review, di- gest, in which one finds for each book title a series of references and selections from published reviews, critical » and otherwise. Certain indexing publications periérm a similar function. Some time ago I became concerned with the problem of developing a cita- tion code for science. This was necessary for the efficient manipulation by mechan- ical devices of entries to scientific indexes. In the course of this research I developed a very simple system for identifying an individual scientific article that lad ap- peared in the periodical press. The re- sulting numerical code consisted of two parts. The first part was a serial number, used instead of an abbreviation, to iden- tify each periodical; it was similar to the serial numbers employed in the World List of Scientific Periodicals, by no means a new idea, For example, Die Biblio- graphie der fremdsprachigen Zeitschrif- ten Literatur has for many years used such a system to save space. The second part of the code number was also a serial number, assigned to each article in a particular publication, start- ing with | and continuing throughout all volumes. The code thus gives no indica- tion of year or volume number, a serious shortcoming. The article number is also not unique, having been used by the Pro- ceedings of the Society for Experimental Biology and Medicine since its inception. These two serial numbers taken together, it can be seen, can identify any published periodical article: It soon became appar- ent, after such codes had been utilized on an experimental basis, that the use gf the codes would facilitate the compilation of a citation index. (Other coding systems would be equally applicable. ) A citation index to science would have the following main characteristics. First there would be a complete alphabetic listing of all periodicals covered, in addi- tion to the code number for each periodi- cal. This list would be similar to the World List, but without the library hold- ings information. The main portion of the citation index would list in straight numerical order the code numbers for all the articles covered. Under each code number, for example, 3001-6789, there 2 would be listed other code numbers rep- resenting articles that had referred to the article in question, together with an in- dication of whether the citing source was an original article, review, abstract, review article, patent, or translation, and so forth. In effect, the system would pro- vide a complete listing, for the publica- tions covered, of all the original articles that had referred to the article in ques- tion. This would clearly -be particularly useful in historical research, when one is trying to evaluate the significance of a particular work and its impact on the literature and thinking of the period. Such an “impact factor” may be much more indicative than an absolute count of the number of a scientist’s publica- tions, which was used by Lehman (3) and Dennis (4). The “impact factor” is similar to the quantitative measure ob- tained by Gross (5).in evaluating the relative importance of scientific journals, a method later criticized by Brodman (6) but used again by Fussler (7). Other advantages would also obtain. In a way such listings would provide each scientist with an individual clipping serv- ice. By referring to the listings for his article, an author could readily deter- mine which other scientists were making reference to his work, thus increasing communication possibilities between sci- entists. It is also possible that the indi- vidual scientist thus might become aware of implications in his studies that he was not aware of before. Most authors like to see how their works are received. Bringing together all book reviews and abstracts is very im- portant, for it is not possible for an author to keep up with the thousands of publications in which his contribution might be reviewed. This applies equally to publishers, It would not be impossible to include books in the citation index. In- deed, as a first suggestion, the use of Library of Congress card numbers as the identifying code for books would seem appropriate. It is necessary next to discuss some realistic questions concerned with the realization of such an index. Bitner (8) has estimated that 30,000 cases are cov- ered by Shepard’s Citations in 1 year, the cases and articles appearing in not more than a few hundred publications. In 1953 about 1 million citations were added—close to 40 citations per case. What is the prospect in scientific litera- ture? The last published edition. of the World List of Scientific Periodicals con- tained more than 50,000 titles in science and technology. It is variously estimated that between 1 and 3 million new scien- tific articles are published each year. The Journal of the American Chemical So- ciety alorie publishes more than 3000 per year, including approximately 2000 origi- nal articles. The order of magnitude is therefore potentially from 50 to 100 times as great as it is for Shepard’s Cita- tions. However, not all of these 50,000 pub- lications are being covered in our present indexing activities, and yet this has not prevented us from continuing indexes of standard type or from starting new ones. Lack of complete coverage is not neces- sarily an argument against a citation in- dex. It is in fact an argument in its favor. Coverage could perhaps be limited to the list of periodicals covered by one of the leading indexing services. This approach would, of course, have an immediate dis- advantage... Such a subject selection would mean that less directly related sub- jects of interest would be excluded, and _these are the publications that the indi- vidual is least likely to cover in his own research. It would be necessary to con- sider all the pros and cons in a selective approach and then to determine the pos- sible utility of such a tool, For example, would a citation index to the 1500 peri- odicals covered by the Current List of Medical Literature be of real value, or, . sirhilarly, a citation index to the 5000 periodicals covered by Chemical Ab- stracts? The Current List would, in fact, offer a good starting point, since it al- ready provides a unique code for the 100,000 items indexed by it each year. Presumably these are the most significant contributions in the covered fields for the year. If 10 is the number of references in the average article, then about 1 million citations would be involved. The prepa- ration of that number annually is not un- reasonable. Shepard’s has already used well over 50 million citations in its pub- lishing activities. The ultimate success of a citation in- dex would depend on many factors. For example, if each periodical would assign unique code numbers to the articles pub- lished, it would be possible for authors to list these numbers in their bibliogra- phies and, thus, to save the work of cod- ing on the part of the citation index staff. It is unlikely that such a development could take place in less than 5 or 10 years, but it is comparable to the problem of getting publishers to include Library of Congress card numbers in their pub- lications. When such a large volume’ of data is to be handled, mechanical devices of high speed and versatility could be used to great advantage and would probably determine success or failure, Once the coding is done, compilation itself is quite mechanical. This could be doné by means of conventional filing slips; the Shepard organization itself has used them success- fully for 80 years. However, it would be facilitated by a mechanical approach using punched cards, The utility of a citation index in any field must also be considered from the point of vicw of the transmission of .ideas. A thorough scientist cannot be satisfied merely with searching the litera- ture through indexes and bibliographies if he is going to establish the history of an idea. Ife must obviously do a great deal of organized, as well as eclectic, reading. The latter is necessary because it is impossible for any one person (the indexer) to anticipate all the thought processes of a user. Conventional subject Jndexes are thereby limited in their at- tempt to provide an ideal key to the lit- erature. The same may be said of classi- fication schemes. In tracking down the origins of an idea, the citation index can be of real help. This is well illustrated by an example from my own experience. Many years ago the Radio Corporation of America developed a reading-aid for the blind (9). This device had an elec- tronic system for converting printed let- ters into recognizable sound patterns. Using the device, a blind mfan could scan a printed page; in a set of headphones he could hear a series of sound patterns, cach letter having its own recognizable sound pattern. In effect, the words were spelled out, letter by letter, in code. I was particularly interested in this device because I had been independently work- ing on a device that would copy print, letter by letter, and reproduce it for bibliographic and other purposes. The two devices had something in common.in that they both employed scanning de- vices. I then wanted to learn whether anyone had ever suggested that the RCA reading-aid could be used for this pur- pose. It will be apparent that if anyone had known of the RCA device and had thought of adapting it for copying pur- poses, a reference to the article might have been made. This reference could easily have been included in an article or patent that was not at all related to the problem of reading devices. A cita- tion index would have given me‘ just what I was after. Nothing could substi- tute for extensive reading, but a great . deal of time could have been saved by bringing the appropriate works to my at- tention. [Tn the course of my reading I did find a few references to this device, one in a book (70), and several others in peri- odical articles, one of which was a Ger- man article on the mechanization of philological analyses and concordance building. The latter article (17) did not discuss my own special interest in copy- ing devices, but it did show the similarity between the author’s and my own think- ing from the point of view of letter-rec- ognition devices, which is what the RCA device attempts to be. In other words, both of us were interested in this device as a letter-recognition device for the analysis of text. In another instance the RCA article was unexpectedly cited in the journal Electronic Engineering in an article on information theory (72) that I was read- ing because of an entirely different in- terest. No subject indexer could have an- ticipated this crossbreeding of interests. Perhaps there are many other articles and books unknown to me that have made similar references to this device. Ifow can they be located when the main subject matter of the article is, on the surface, so unrelated in nature? One might say that it would be pos- sible to index articles more thoroughly to achieve the same results. For example, the article on information theory, if thor- oughly indexed, might have included an entry under reading devices for the blind. Yet if this were done, our periodical in- dexing services would clearly become hopelessly overloaded with material that is not necessary to lead us to the micro unit—the entire article or one of its major sections. Although it might be said that no scientist interested in the greater comprehensiveness to be found in a cita- tion index would object to having such a great mass of references in a subject in- dex, this is impracticable. It would re- quire an army of indexers to read the articles and identify the exact subject matter of every paragraph or sentence. Yet this would be necessary. To illustrate, it is only in the very last paragraph of the article on information theory that one would find a reference to reading devices for the blind. Were an army of indexers available, it is still doubtful that the proper subject indexing could be made. Over the years changes in terminology take place, that vitiate the usefulness of a standard sub- ject index. To a certain extent, this is overcome through the citation approach, for the author who has made reference to a paper 40 or 50 years old has inter- preted the terminology for us. By using authors’ references in compiling the cita- tion index, we are in reality utilizing an army of. indexers, for every time an author makes a reference he is in effect indexing that work from his point of view. This is especially true of review articles where each statement, with the following reference, resembles an index entry, superimposed upon which is the function of critical appraisal and inter- pretation. To the indexer this has its ad- vantages as well as its disadvantages (13). To determine in a practical way what the citation index could offer, it was de- cided to track down the citations made in one journal to a single significant ar- ticle, in order to compile a sample entry for the citation index. At the suggestion of Erich Meyerhoff, I selected Hans Selye’s famous article on the general adaptation syndrome (/4). A systematic search was then made of all papers that were published in the Journal of Clinical Endocrinology subsequent to Selye’s paper up to 1951—a period of 5 years, including well over 500 articles. Every bibliography in each of the 500 articles was checked for a reference to Selye’s article. Twenty-three articles wete found to make such reference; each of them was then checked for the character of the information provided. Examination of the citation list (Table !) shows the great variety of subject mat- ter included. One thing became quite clear, even to the uninitiated—that is, the influence of Selye’s article has been quite pronounced. Such evidence is extremely valuable to the historian. It is interesting to note that, although all the articles cited were indexed in Quarterly Cumulative Index Medicus, not one is to be found there under the heading “Adaptation.” In fact, it is sur- prising not to find any articles from this journal under this subject heading. It also becomes quite obvious that many references to Selye’s paper were general and contribute little or nothing Table 1. Index sample based on article by Hans Selye, “General adaptation syn- drome” [f. Clin. Endocrinol. 6, 117 (1946)]. The code number for this journal in the World List is 11,123a; the article number is arbitfarily taken as 687; and the code number for the article is 11123a- 687. The 23 articles that cited Selye’s article are listed, followed by A hypo- thetical citation index entry for Selye’s articles R, review article; A, abstract; O, original article. 1, Williams, R. H.: Thyroid & Adrenal Interrelations, 7: 52-57 (1947). 2. Wenning, E. H.: Glycogenic Corti- coids, 7: 79-101 (1947). 3. Forbes, ef al.: 17-Ketosteroids in Trauma and Disease, 7: 264-288 (1947). 4. Talbot, et al.: Excretion of 11-Oxy- corticosteroids, 7: 331-350 (1947), 5. Castillo, E. B. del, ef al.: Syndrome of Rudimentary Ovaries, 7: 385-422 (1947), 6. Forsham, P. H., et ai.: Adrenocorticotropin, 8: (1948). 7. Pincus, G., et al.: Rhythm in Biped Excretion, 8: 221-226 (1948). 8. LeCompte, P. M.: Width of Adrenal Cortex in Lymphatic Leukemia, 9: 158-162 (1949). 9. Wolfson, W. Q.: 17-Ketosteroids in Gout, 9: 497-513 (1949). 10. Stein, H. J., et al.: Hormonal Re- sponse to Heat and Cold, 9: 529-547 (1949). 11. Davis, M. E.: Eosinophils in Preg- nancy and Labor, 9: 714-724 (1949). 12. Conn, J. W.: Na and Cl of Sweat as Cortical Index, 10: 12-23 (1950). Pituitary 15-66 13. Recant, L., é¢ .al.: Effect of Epine- phrine on Eosinophils, 10: 187-229 (1950). 14. McArthur, J. W., ef al.: Urinary Excretion of Corticosteroids in Dia- betic Acidosis, 10: 307-312 (1950). 15. Bors, ‘E.: Fertility in Paraplegic Males, 10: 381-398 (1950). ‘16. Grossman, S., et al.: Idiopathic Lac- tation following thoracoplasty, 10: 729-734 (1950). : 17. Cooper, J. S., ef al.: Metabolic Con- sequences of Spinal Cord Injury, 10: 858-870 (1950). 18. Hioco, D.: Adrenal Metabolites in Bronchial Asthma, 10: 1570-1578 (1950). 19. Jailer, J. W.: Pituitary-Adrenal Sys- tem in Infants, 11: 186-192 (1951). 20. Deane, H. W.: The Adrenals in Ex- perimental Hypertension, 11: 193- 208 (1951). 21. Hioco, D., et al.: Epinephrine and ACTH in Bronchial Asthma, 11: 395-407 (1951). 22. Schaffenberg, C. A., et al.: p-Hy- droxypropiophenone (PHP) and other so-called pituitary inhibitors, 1: 1215-1223 (1951). 23. Talbot, N. B., e¢ al.: Urinary Water- Soluble Corticosteroids, 11: 1223- 1236 (1951). Citation Index Entry 11123s-687 464-9789(R) 869-3366(R) 1105-9876(A) 1123-4432(R) al1,123-0752(O) -0779(O) -7264(O) -7331(O) -7385(O) -0866(O) -8221(O) -9158(O) -9497(O) -9529(O) to the readers’ enlightenment, since exact page references are not provided. In sev- eral cases the Selye article is even. cited but not referred to in the text. Selye’s in- fluence on all of these authors is quite apparent. In particular instances the ci- tations are of value in locating confirma- tory evidence of some of Selye’s claims. Thus, in the case of a highly significant article, the citation index has a quanti- tative value, for it may help the historian to measure the influence of the article— that is, its “impact factor.” With regard to a less significant work, one would sus- pect that the bibliographic advantages might be increased, because the scientist or librarian would be provided with ref- erences not to be found in conventional indexes. The preliminary evidence pre- 4 sented indicates that the citation index offers interesting possibilities for another approach to bibliographic control. The next step in compiling the index for the Selye article would be to seek out additional references to it in more periph- eral journals, but obviously the farther away you get from the immediate subject area of the main article, the fewer the references to it you will locate. Yet these may well be the most useful references of all, for the cross-fertilization of subject fields is one of our most important prob- lems in science literature, It will be well to close with a brief description of how the citation index might be compiled. The first step would be. the selection of the particular group of periodicals to be covered; next, the period to be covered, say, only that since 1900. The problem actually has two facets: the selection of periodicals to be covered in order to obtain citations, and the selec- tion of those articles for which we want a citation record. For example, all ar- ticles in journals in the Current List of Medical Literature that have remained in continuous publication since 1900 might be coded, in which case the Jaur- nal of Clinical Endocrinology would not be included. However, we might include as citation sources all journals covered by the Current List. Thus, the bibliogra- phies appearing in articles in the Journal of Clinical Endocrinology would supply references to the basic group of articles. Each coder would be assigned a group of articles in a particular journal. The first step would be to number each article in the journal in ascending order, by utilizing a complete table of contents of that journal from its inception. Once a code number has been assigned to each article, the proper codes may then be assigned to each periodical. This might be the number given in the World List, with new numbers for any periodi- cals not to be found there. Actual coding starts with the first ar- ticle in a particular periodical. The coder prepares a 3- by 5-in. card for each cita- tion made in the article. Each card should give (i) the code number for the citing article, (ii) the code number for the article cited, and (iii) a classification of the citing article as an original con- tribution, review article, abstract, and so forth. Many references will be excluded by the limits of coverage set up. Thus all references to articles not in the prescribed list of journals would be excluded. All books would be excluded unless. otherwise specified, in which case the reference card would carry. the code for the citing article and the code for the book (its LC card number). After all the articles had been coded, it would next be necessary to.sort the cards by the code numbers for the items cited. This would yield a group of cards for each cited article. These would then be sorted by code numbers for the citing articles, This completes the coding and sorting. The next step would be prepara- tion for the printer. From this description it will be appar- ent that, although a great volume of ma- terial is to be covéred, relatively unskilled persons can perform the necessary coding and filing. Professional supervision would still be required, because certain decisions require skilled judgment, for example, ‘when ibid. or loc. cit. must be carefully interpreted, Footnotes tend to make cod- ing somewhat cumbersome. The code I have described is merely an example used to illustrate the method in principle. If the system Were adopted, then in the future every author ought to be required ‘to include the serial number of each item he referred to, so as to facilitate not only the compilation of citation indexes but also other operations such as requests for reprints (15, 16). _ In a certain sense a citation index is not very different from a compendium like Beilstein, which gives a rather com- ptete record of a compound, compiled by a similar method. A citation index for the literature of chemistry would un- doubtedly make the preparation of such works as Beilstein much easier than it is at present. The new bibliographic tool, like others that already exist, is just a starting point in literature research. It will help in many ways, but one should not expect it to solve all our problems. References and Notes 1. P. Thomasson and J. C. Stanley, Science 121, 610 (1955). Thomasson and Stanley were com- menting on C. Zirkle’s discussion of the use of fraudulent data [Science 120, 189 (1954}). . W. GC. Adair, Am. Documentation 6, 31 (1955). iy 3, H. G. Lehman, Sci. Monthly 78, 321 (1954). 4. W. Dennis, ibid. 79, 180 (1954). 5. P. L. K. Gross and E. M. Gross, Science 66, 385 (1927). 6. E. Brodman, Med. Library Assoc. Bull, 32, 479 (1944). 7. H. H. Fussler, Library Quart. 19, 19 (1949). 8. H. Bitner, personal communication, April 1954. 9. V. K. Zworykin and L. E. Flory, Proc. Am. Phil. Soc. 91, 139 (1947). 10. R. R. Shaw, Machines and the Bibliographt- cal Problems of the Twentieth Century (Univ. of Illinois Press, Urbana, 1951) p. 19. (Re- printed from Bibliography in an Age of Science (Univ. of Illinois Press, Urbana, 1951). 11. R. Busa, Nachr, Dokumentation 3, 14 (1952). 12. A. M. Andrew, Electronic Eng. 25, 471 (1953). 13. E. Garfield, “Review literature as a source of critical entries for scientific indexes, unpub- lished paper, December 1952. 14, H. Selye, J. Clin. Endocrinol. 6, 117 (1946). 15. E, Garfield, Science 120, 1039 (1954). 16. J. A. Behnke, Science 120, 1055 (1954).