Jointly published by Elsevier Science Ltd, Oxford and Akadémiai Kiadó, Budapest
Scientometrics ,Vol. 43, No. I (1998) 69-76
(Received April 9, 1998)

(Comments on Theories of Citation? L. Leydesdorff, Scientometrics, 43(1998) No:1)

Chairman Emeritus
Institute for Scientific Information,
3501 Market Street,
Philadelphia, PA 19104 (USA)
    The Scientist,
3600 Market Street,
Philadelphia, PA 19104 (USA)

Theories of citation are as elusive as theories of information science, which have been
debated for decades. But as a basis for discussion I offer the term citationology as the theory and
practice of citation, including its derivative disciplines citation analysis and bibliometrics.
Several maxims, commandments if you will, have been enunciated. References are the result of a
specialized symbolic language with a citation syntax and grammar. References, like words, have
multiple meanings which are related to the aposteriori quality of citation indexes. Therefore,
citation relevance cannot be predicted. Mathematical microtheories in bibliometrics abound,
including the apposite laws of scattering and concentration. Citation behavior is a vast sub-set of
citation theory, which like citation typology, can never be complete. Deviant citation behavior
preoccupies certain authors but it is rarely significant in well-designed citation analyses, where
proper cohorts are defined. Myths about uncitedness and the determinants of impact are
discussed, as well as journal impact factors as surrogates and observations on scientists of
Nobel Class.

After two years at Johns Hopkins investigating "machine documentation," and another year
as a student of library science, I became, fortuitously, a documentation consultant By 1954, I
called myself an information engineer, which was an apt description of my professional
consulting activities However, Pennsylvania licensing law requires that engineers be graduates
of engineering schools So I became an information scientist! I’ve never thought of myself as an
information theoretician and have been skeptical about a need for a theory of information
science I’ve practiced information science and engineering without explicit theoretical support.
But undoubtedly there are underlying principles which can guide information scientists who, like
myself. could be called "citationists" or "citationologists." If there is a theory and practice of
citation, it should probably be called citationology.

I myself did not begin as a bibliometrician, scientometrician, or citation analyst.
There was no guiding “theory” which led to my interest in citation indexing. My basic
dissatisfaction with traditional methods of indexing, cataloging and classification was a
major stimulus. Then my serendipitous encounter with the American system of legal
documentation led to citation indexing. Citators, as they were called, provided the
framework for my earlier parallel interest in natural language and “simple” systems for
dissemination. The latter were manifested in the creation of Current Contents - which
eventually led to title-word, and permuterm indexing.1Nevertheless, a formalized
description of citation indexes might be regarded as essential to citation theory
building. In their printed form, citation indexes are two-dimensional displays of the
linkages between document addresses. These addresses are sometimes called
references, but also citations, depending upon the direction of reference-to-reference
links. Papers and books cite references, that is, earlier papers and books and other
documents (patents, letters, etc.) but occasionally cite into the future, for example,
papers and books in press. Citations to earlier work provide backward links while
citation indexes provide forward links.

From the outset, we recognized that references or citations reflect a natural
international language of science and scholarship. The “grammar” of bibliographic
citations was described by compiling a dictionary of many dozens of citation formats.2
Their symbolic role was formally described by Small in his now seminal paper.3 It is
relevant to mention my earliest lecture tours. To illustrate the symbolic role of citations,
I often quoted Lewis Carroll’s “Humpty Dumpty” to express the ambiguous nature of
words and citations. “When I use a term it means just what I want it to mean - nothing
more or less.” And so when you use a cited reference (citation), it also means what you
want it to mean. A citation is generally more precise than words, but its meaning is
ambiguous nevertheless. We all use citations with slightly different intentions and
meanings. In some contexts citations can be extremely precise symbols. If you see a
reference to Oliver Lowry’s 1951 classic on protein determination,4  you can be 99.9%
certain that his method. is being used, but you cannot predict whether or when a slight
modification is reported by the 250,001st paper that has cited it. From the beginning,
citation indexes were characterized as aposteriori indexes, in contrast to apriori
traditional indexes. Wouters has recently reminded us of this quality of citation

I would like to enunciate some commandments or maxims of a theory of citation. A
first commandment might be - there is no way to predict whether a particular citation
(use of a reference by a new author) will be “relevant." Cleverdon's great contribution to
information retrieval theory6  was built around the notions of precision, recall, and
relevance. But none of his or related studies accounts for or measures relevance in
retrieval by citation. As in so many things, relevance is in the eye of the beholder. The
main theme of most papers that cite the Lowry method have little to do with the method
itself. Relevance studies concern themselves primarily with main theme indexing.
In this respect, related records, a variant on bibliographic coupling,7 produces a type
of relevance ranking which might be compared with ranking by permuterm indexing,
that is co-word occurrence.

Another maxim -- if author X cites the work of author Y, regardless of the reason,
then this fact alone makes the citing paper relevant to author Y and, furthermore, author
X may be interested in other papers that cite Y. Undoubtedly, Professor Lowry long ago
gave up trying to assess the thousands of papers that cited his work each year, but each
occurrence was highly “relevant” to others interested in protein determination methods
and reagents.

I have experienced the greatest excitement in finding references to my own work in
the least likely of places, as e.g., Kevin Kelly's discussion of citation analysis8
and, more recently, in Candace Pert’s reflection on - Current Contents - in The Molecules
of Emotion.9 Conventional subject headings or title word indexing would not call out
these connections.

Full-text searching will make this possible. While the use of bibliographic coupling
(related records) can give some degree of relevance ranking to the papers that cite a
particular paper I have written, it is often the “onesies” that are the most interesting. In
other words, the degree of linkage between my work and the newly discovered author
is simply that we both have cited one or two other authors in common.
Putting aside the anomaly of Oliver Lowry and other super citation stars, one can
say, for the sake of theory building, that whenever author X is cited, he will regard the
citing work as initially relevant, even though on closer inspection it may not prove
interesting. However, in laboratory-based scientific research, it is highly likely that high
degrees of bibliographic coupling (related records) will produce high degrees of

Is there a body of laws which govern the citation world? The literature of
bibliometrics provides all sorts of mathematical descriptions of citation distributions.
These go back quite far, and I will not attempt to recapitulate these microtheories. We
all know or have heard about Lotka’s Law, Pareto’s Law, Zipf’s Law, or whatever, but
also especially Bradford’s Law of Scattering. The latter has been discussed in countless
papers by Brookes, Bookstein, Price, Leimkuhler, Rousseau and others too numerous to
mention. Bibliometricians are fascinated by these mathematical exercises which permit
them to display their admirable mathematical and probabilistic insights.

My limited contribution to this mathematical microtheory world includes my
counterpoint to Bradford’s Law of Scattering,10 namely Garfield’s Law of
Concentration.11  It seems remarkable to me after so many decades that so few people
really appreciate the economic consequences of these phenomena. The most productive
journals have remarkable stability and impact. This has been demonstrated periodically
every decade since we began our first experiments with the Genetics Citation Index.
When we created, restrospectively, the 1945-54 Science Citation Index, - we selected
the core group of 600 journals by a purely algorithmic procedure - that is citation
frequency. The effectiveness of this list is demonstrated daily. This group of journals
identifies a high percentage of the post-war literature which was then and still is cited
regularly. .

Another modest contribution I made to the microtheory of citation is Garfield’s
constant.12 Actually, we know that this “constant” is really a ratio. That ratio is
remarkably “stable” considering how much the literature has grown. Due to continuous
growth of source journal coverage and increasing references cited per paper, the ratio of
citations to published papers increased about 75% from 1945 to 1995 - from 1.33 to
2.25 over the past 50 years. It is the inflation of the literature which increases the ratio
each year.

Citation Behavior
If description is part of citation theory building, the characterization of citation
behavior must be part of it. A considerable literature discusses citation “behaviors.”
Such behavior was discussed in the earliest days of experimenting with citation index
building which I recently reviewed in “When to Cite"l3 There probably never will be a
complete typology of citation behavior. There always will be new reasons why people
cite. My 1979 text on Citation Indexing,14 contained a somewhat limited typology
which is sufficient for most purposes. Within each branch of science, there will be more
specific types of citation. For example, in chemistry, an author may cite a paper simply
because it reports the melting point of a solid chemical. One can never know apriori whether
or how that citing paper will interest another chemist. The latter might cite the same paper because
it mentions the failure to make a similar compound. That is why citation indexes can be
useful to the compilers of specialized chemical databases or handbooks which provide
precise information of this kind in a more condensed form. It also highlights the
dilemma faced by citation index compilers who regularly encounter pageless
documentation.l5 The latter simply means that a book or paper has been cited but the

specific page or chapter has been omitted from the reference or is buried in the text of
the citing paper. The dilemma lies in whether or not these pageless citations should be
unified with page specific citations so that all citations to the main work are combined
into a single entry. Otherwise, scientometricians obtain distorted citation counts
especially when they rely on electronic searches that do not unify the variations.
Deviant citation behavior preoccupies critics of citation analysis. It is fashionable
for them to mention that authors are lax in their scholarship even with good refereeing.
Anecdotal accounts would have us believe that most documentation is fraudulent. But I
have never seen a documented case of a citation analysis gone astray due to
accumulated deviant citation behavior. A common flaw in appraising citation analysis,
is that author citation frequency is used out of historical context. Using the current
year’s literature to ascertain the most influential authors of the past is naive indeed.
Such an exercise ignores another maxim of citation analysis. Always measure
citation links in the appropriate period of literature. Cawkell and I addressed these
issues in discussing the long-term influence of Albert Einstein and other authors. 1617
It is absurd to use only recent literature for such purposes. Longitudinal data is essential
to evaluate the historical influence of a particular paper, author or book. It is often
forgotten how rapidly the scientific literature changes and how new discoveries are
superseded. In short, the modern scientific article makes no pretense at being
historically comprehensive and stresses the most recent literature. Modem
bibliographies do not recapitulate the entire literature of a topic unless that is the stated

A theory of citation might include a set of commandments of citation analysis.
Another commandment that pertains to the evaluation of people, journals, and
institutions - always compare or judge equivalent or truly comparable cohorts. Naive
administrators, uninformed in citation analysis, will make the mistake of using citation
data without regard to the discipline or invisible college involved. Cross-disciplinary
comparisons are usually inappropriate. Even in large disciplines, it can be difficult to
establish perfect cohort groups of authors or journals. To approach ideal cohorts, co-word
and/or co-citation clusters can be used. Thus, when we wanted to identify the
pioneers of apoptosis we created a database of papers whose titles contained the word
apoptosis or its equivalent, programmed cell death.18 And even then the omission of a
paper using the hyphen in cell-death adversely affected the original choice of core

A theory of citation may also need some maxims concerning the many myths about
citation studies. For example, it is repeatedly asserted that the size of a discipline is the
primary determinant of the impact factor for people, papers, or journals. But in
discussing Garfield’s Constant I demonstrated12,  that it is the average number of
references cited per paper (R/S) citation half life, and utilization factors that determine
impact - and not the size of the literature.

The size of the literature will determine the number of papers that can exceed a
particular citation threshold. Thus, biochemistry will produce a large number of papers
whose citation frequency exceeds 400 or 500 citations, what I’ve called citation
classics.20 But the higher impact of average biochemistry papers is due to the field’s
high R/S per paper rather than the number of papers in the field.

The considerable variation in citation frequency of articles in large fields is
overlooked simply because we are conscious of the many highly-cited classics. We are
unable to be fully conscious of the thousands of papers that are less frequently cited.
The large number of less frequently cited papers is difficult to visualize, unless one
examines an article-by-article ranked listing for a specific journal.

But even a paper or book in a small field that attracts cross-disciplinary citation may
break out of the expected impact of that field. Kuhn’s Structure of Scientific Revolution
is a superclassic even though it represents a relatively small field of scholarship.
There is also the uncitedness myth. Hamilton21 helped to perpetuate this myth by
misinterpretation of citation data which was undifferentiated with respect to types of
editorial material. Pendlebury effectively rebutted his data 22 but Hamilton’s claims
persist due to the high visibility of the journal in which it was published and because
the theme has popular appeal. Wade repeated the myth in a recent New York Times
story.23?  While I myself have often reported that a large number of papers do indeed
remain uncited, they are primarily published in low impact journals. Thus, another
commandment in citation studies is: Thou shalt compare items in equivalent editorial
categories such as original research papers, reviews, letters, etc.

Another commandment, concerns the journal impact factor. This ratio, which was
created to facilitate the comparison of journals regardless of size. has lately been used
as a surrogate for actual citation data on individuals. There has been a spate of articles
deploring this use but many of the authors have used this particular malpractice as an
excuse to malign all quantitative studies. Seglen has justifiably criticized evaluation
exercises which blindly use journal impact factors as surrogates. It is well known that
there is considerable variation in citation frequency of articles within the same

The current impact factors reported in ISI’s Journal Citation Reports - are useful in
their appropriate place. Long-term cumulative impact data can also be used. Various
techniques for combining current impact with half-life to produce estimates of long-term
impact have been made. The recent availability of ISI’s Journal Performance
Indicators makes estimation unnecessary in most cases. The calculation of long-term
impacts will, however, not produce exceptional results unless one is making cross-disciplinary
comparisons.26 The rankings of journals within their appropriate category
does not change all that much but generalizations are to be avoided.

As a concluding note in these miscellaneous comments, let me refer to some data on
Nobel Class scientists which was reviewed at the AAAS meeting in Philadelphia.27 As
Sher and I reported in 1965, the average Nobel Prize winner published five times the
average author and were cited 30 times the average.28 Since that long-ago study, we
also found that over 95% of Nobel Prize winners are authors of one or more Citation
Classics. By extension, over 50% of the 1,000 most-cited scientists are members of the
U.S. National Academy of Sciences. Further, these scientists “of  Nobel Class” are
among the most-cited decile for their discipline. The general figure of 50% fits well
with the anecdotal account of a former Academy president who told me “for every
scientist elected to the Academy, there is another equally qualified who is not elected.”
As Zuckerman expressed it,29in characterizing the 40 member limit of the French
Academy, “Who shall occupy the 41st chair?”

Conclusion: a citation-ranked list of scientists will identify 50% or more present and
future members of the Academy. Therefore, while citation frequency alone does not
warrant election, the nominating group should at least consider as candidates all those
who achieve a given threshold of citation frequency.

1. back to text GARFIELD, E., The Permuterm Subject Index :An Autobiographical Review,
    Journal of the American Society of Information Science , 27(5-6):288-91 (1976).
    Reprinted in Essays of an Information Scientist,Volume 7. Philadelphia: ISI Press, p. 546-550, 1985.

2. back to text GARFIELD, E., SHER, I., Progress Report of Citation Index Project,
    October 9, 1962 (unpublished).

3. back to text SMALL, H. G. Cited Documents as Concept Symbols, Social Studies of Science ,
    8(3):327-340 ( 1978).

4. back to text LOWRY, O. H., ROSEBROUGH, N. J., FARR, A. L., RANDALL, R. J.,
    Protein Measurement with the Folin Phenol Reagent, Journal of Biological Chemistry,
    193:265-27 5 (195 1).

5. back to text WOUTERS, P., The Signs of Science, Scientometrics, 41(l-2):225-241 (Jan-Feb 1998).

6. back to text CLEVERDON, C. W., Cranfield Tests on Index Language Devices,
    ASLIB Proceedings, 19: 173 (1967).

7.back to text KESSLER, M. M., Bibliographic Coupling Between Scientific Papers,
    American Documentation, 14(1):10-25 (1963).

8.KELLY , K., Out of Control: The New Biology of Machines, Social Systems and
    the Economic World, Addison-Wesley, 520 p., 1995.

9. back to text PERT, C., Molecule of Emotion Why you Feel the Way you Feel, Scribner, 304 p.. 1997.

10 a. back to text  BRADFORD, S. C., Sources of Information on Specific Subjects, Engineering, 137:85-86 , 1934.
         b. BRADFORD, S. C., Documentation, Washington, DC: Public Affairs Press. 1950, 156 p.

11. back to text GARFIELD. E., The mystery of the transposed journal lists -
     wherein Bradford’s law of scattering is generalized according to Garfield’s law of concentration,
    Current Con tents, No. 7 :5(August 4, 1971)
     Reprinted in Essays of an Information Scientist, Volume 1, Philadelphia: ISI Press, p. 222-223, 1977

12.  back to text GARFIELD, E., Is the ratio between number of citations and publications cited a true constant?
    Current Contents. No. 8: 5-7 (February 9, 1976).
    Reprinted in Essays of an Information Scientist, Volume 2. Philadelphia: ISI Press, p. 419-421, 1977.

13. back to text GARFIELD, E., When to Cite, Library Quarterly, 66(4): 499-558 (October, 1996).

14. back to text GARFIELD, E., Citation Indexing - Its Theory and Application in Science,
    Technology and Humanities, Philadelphia: ISI Press, 1979. p. 274.

15. back to text GARFIELD, E., Pageless documentation; or what a difference a page makes,
    Current Contents, No.17:3-6 (April 29. 1985). Reprinted in Essays of an Information Scientist,
    Volume 8, Philadelphia: ISI Press, p. 160-163. 1986.

16.  back to text GARFIELD, E., The Einstein Centennial and Citation Analysis,
    Current Contents, No. 17: 5-9 (April 27, 198 I).
    Reprinted in Essays of an Information Scientist, Volume 5. Philadelphia: ISI Press, p.91-95, 1983.

17. back to text CAWKELL A. E., GARFIELD, E., Assessing Einstein’s impact on today’s
    science by citation analysis, (GOLDSMITH M, MACKAY A., WOUDHUYSEN J., (Eds),
    Einstein: the First Hundred Years, Oxford: Pergamon Press, Pg. 3 I-40, 1980.
    Reprinted in Essays of an Information Scientist, Volume 5. Philadelphia: IS1 Press, p. 9 l-95, 1983.

18. back to text GARFIELD, E., MELINO G., The growth of the cell death field:
    An analysis from the IS1 Science Citation Index, Cell Death and Differentiation, 4(5):352-361 (July, 1997).

19. back to text GARFIELD, E., MELINO G., The growth of the cell death field: An analysis from the
    ISI Science Citation Index -- erratum, Cell Death und Differentiation 5( 1): 127 (January, 1998).

20. back to text GARFIELD, E., Introducing Citation Classics: the Human Side of Scientific Reports,
    Current Contents, No. 1. p. 5-7 (January 3, 1977).
    Reprinted in Essays of an Information Scientist, Volume 3. Philadelphia, ISI Press, p. 1-2, 1980.

21. back to text HAMILTON , D. P., Publishing by - and for? - the numbers,
    Science, 250(4986): 1331-1332 (December 7, 1990).

22.back to text PENDLEBURY, D. A., “Uncitedness”. Letter to the Editor, Science, 25 l(5000):
    1410- 1411 (March 22, 1991).

23. back to text WADE, N., No Nobel Prize This Year? Try Footnote Counting, New York Times, p. F4 (October 7, 1997).

24. back to text SEGLEN, P. O., Why the Impact factor of journals should not be used for evaluating research,
    British Medical Journal, 314(7079).498-502 (February 15, 1997).

25.back to text SEGLEN, P. O., Evaluation of Scientists by Journal Impact, Representations of Science and Technology;
    Proceedings of the International Conference on Science and Technology Indicators, Bielefeld,
    Bielefeld, 10-12 June, 1990. P. WEINGART. R SEHRINGER, M. WINTERHAGER, (Eds),
    Leiden, DSWO Press, p. 240-252, 1992.

26. back to text GARFIELD, E., Long-Term Vs Short-Term Journal Impact: Does It Matter?
    The Scientist, 12(3). 11-13 (February 2, 1998).

27. back to text  GARFIELD, E., Mapping the World of Sctence. Paper presented at the 150th anniversary meeting of the
    AAAS, Philadelphia, PA, February 14, 1998.
    (see http://www.the-

28.back to text SHER,I. H., GARFIELD , E.. New tools for Improving and Evaluating the Effectiveness of Research.
    Research Program Effectiveness, M C. Yovits, D. M. Gilford, R. H. Wilcox,  E. Staveley.
    H. D. Lemer (Eds), New York: Gordon and Breach, p. 135-146. 1966.

29. back to text ZUCKERMAN. H., The Scientific Elite. Nobel Laureates in the United States, New Brunswick and
    London: Transaction Press, 1996.

12.  back to text GARFIELD, E., Is the ratio between number of citations and publications cited a true constant?
    Current Contents. No. 8: 5-7 (February 9, 1976).
    Reprinted in Essays of an Information Scientist, Volume 2. Philadelphia: ISI Press, p. 419-421, 1977.