First part of AURA study of publications in astronomical journals for the first decade of the

49 Pages·2010·2.6 MB·English

by  Jay A. Frogel

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Astronomy’s Greatest Hits:
The 100 most Cited Papers in Each Year of the
st
First Decade of the 21 Century (2000 – 2009)

Jay A. Frogel

Association of Universities for Research in Astronomy
1212 New York Avenue, NW, Suite 450
Washington, DC 20005

[email protected]

Accepted for Publication in
Publications of the Astronomical Society of the Pacific
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ABSTRACT
The first decade of the 21st Century and the last few years of the 20th have been
transformative for ground and space based observational astronomy due to new observing
facilities, access to digital archives, and growth in use of the Internet for communication and
dissemination of information and for access to the archives. How have these three factors
affected the characteristics and content of papers published in refereed astronomical journals as
well as the journals themselves? In this and subsequent papers I will propose answers to this
question.
The analysis in this the first paper of a series is based on an examination of the 100 most
cited papers in astronomy and astrophysics for each year between 2000 and 2009, inclusive, and
supplemental data from 1995 and 1990. The main findings of this analysis are:
Over the ten-year period the total number of authors of the top 100 articles per year has
more than tripled. This increase is seen most strongly in papers with more than 6 authors. The
number of unique authors in any given year has more than doubled. The yearly number of papers
with 5 or fewer authors has declined over the same time period.
Averaged over the ten-year period the normalized number of authors per paper increases
steadily with citation rank – the most highly cited papers tend to have the largest number of
authors and visa versa. This increase is especially notable for papers ranked 1 through 20 in
terms of number of citations and number of authors.
The distribution of normalized citation counts versus ranking is remarkably constant
from year to year and, except for the top ranked half dozen or so papers in each year, is very
closely approximated by a power law. Nearly all of the papers that show the most divergence
from the power law fit – all in the sense of having a high number of citations – are based on the
results of large observational surveys.
Amongst the top 100 papers there is a small but significant correlation of paper length
with citation rank. More striking, though, is that the average page length of the top 100 papers is
one and a half times that for astronomy papers in general.
For every year from 2000 to 2008 the same 5 journals account for 80 to 85% of the total
citations for each year from all of the journals in the category “Astronomy and Astrophysics” by
ISI’s Journal Citation Reports. These numbers do not include Nature or Science. Averaged over
the 10 year time period studied in this paper, these same 5 journals account for 77% of the 1000
most cited papers, slightly less than the journals’ fractional contribution to the total number of
articles published by all journals. The 5 journals are A&A, AJ, ApJ, ApJS, and MNRAS.
Two samples of the top 100 cited papers, both for the six years 2001 to 2006 but
compiled two and a half years apart, show that a significant number of articles originally ranked
in the top 100 for the year, drop out and are replaced by other articles as time passes. Most of
the drop-outs address topics in extra-galactic astronomy; their replacements for the most part
deal with non-extra-galactic topics.
Finally, some additional findings are noted that relate to the entire ensemble of
astronomical journals published during the Century’s first decade.
Various indicators of internet access to astronomical web sites such as data archives and
journal repositories show increases of between factors of three and ten or more I propose that
there are close complementarities between the communication capabilities that internet usage
enables and the strong growth in numbers of authors of the most highly cited papers. Subsequent
papers will examine this and other interpretations of the analysis presented here in detail.
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1. INTRODUCTION
The first decade of the 21st Century and the last few years of the 20th Century have been
transformative for observational astronomy. Three important reasons are new facilities, the
digital archives that have resulted from them - as well as from older facilities - and the growth in
use of the Internet for communication and dissemination of information and for enabling easy
access to the archives. Intimately linked to all three of these is the steady increase in computing
power as illustrated by the continuing validity of Moore’s law (Moore, 1965).
First, consider the new facilities. On the ground essentially all of the existing 6-meter
and larger telescopes have come into regular operation. In space the wide range of new missions
includes Mars orbiters and landers, the Cassini mission to Saturn, Chandra, FUSE, XMM-
Newton, WMAP, RHESSI, GALEX, Spitzer, Swift, Deep Impact, MESSENGER, Fermi,
Kepler, Herschel, Planck, and WISE as well as Hubble Servicing Missions, and several high
altitude ground and balloon-borne CMB telescopes. Some of these new capabilities have only
just begun operations so they have not yet begun to produce a significant number of publications.
Second, enormous and readily accessible digital databases have resulted from major new
observational surveys at all wavelengths from space and from the ground. Examples of ground
based surveys include the SDSS, 2dF, 2MASS, Auger, HESS, and VERITAS. Some of these,
while not necessarily producing large data sets, do result in highly cited papers. WMAP, Planck,
WISE, and Fermi are new space missions with all sky surveys as their primary goal. And, as
from the ground, targeted surveys on small areas have produced rich databases that are the
subject of intensive study and have resulted in many highly cited publications. These include the
Hubble Deep and Ultra-Deep Fields, GOODS, the several supernova surveys, and the search for
extra-solar planetary systems.
Third, is the phenomenal increase in internet usage. The internet enables electronic
access to nearly the entire body of astronomical literature (eg. Kurtz, et al., 2000 and Henneken
et al., 2009), to most large repositories of data, and it permits essentially instantaneous
communication across all geographical boundaries (cf. Abt 2000b). The internet also gives
access to many tools for manipulating data and mining the astronomical archives.
What effects have these new facilities, data archives, and means of information exchange
had on astronomical publications? In this paper, the first of a series, I will identify trends and
patterns in astronomical publications that will be the basis for gauging these effects. This work
is based on astronomy’s “greatest hits”, the 100 most cited papers in astronomy for each year of
the last decade (2000 to 2009), 1,000 papers, by the top 100 papers for 1990 and 1995.
This paper concentrates on the inter-relationship between properties that characterize
these top-cited papers and their dependence on time. Among the properties that I examine are the
journal in which they are published, the number of authors on a paper, a paper’s citation record,
and page length. For the most part I will postpone interpretation of the findings to a subsequent
paper which will discuss the scientific content of the papers, how, for observational papers, the
data were acquired, and what role data archives played in the research.
Abt (2000b) gave several interesting answers to the question “what can we learn from
publication studies”, and pointed to a number of ways in which such studies can be put to
practical use. The analysis in this paper will provide additional answers to his question that I
hope will be of equal interest.
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2. SOURCES FOR THE DATA AND THE ANALYSIS PROCEDURE
A simple, commonly used quantitative measure of the impact of a published account of a
piece of research is the number of citations to the publication. The h-index (Hirsch 2005) is an
often used statistic that attempts to quantify both the scientific output and impact of individuals
and institutions. Recent examples of citation studies (as well as studies of “citation studies”)
may be found in the work of a number of authors; for example Abt (2000a, 2000b, 2003, 2006a,
b, 2007), Meho (2007), Madrid and Macchetto (2004), Trimble and Ceja (2007, 2008, 2010),
Van Noorden (2010), Kinney (2007, 2008), Kurtz, et al. (2005), Stanek (2008, 2009), White
(2007), and Molinari and Molinari (2008). The goals of these and similar papers have included
not only the evaluation of the impact of individuals, institutions, and telescopes, but also a search
for broad trends in astronomical publications.
The quantitative analysis in the present paper is based on two overlapping samples of
highly cited papers in astronomy and astrophysics, nearly all of which are in peer reviewed
journals, but about 5% are in journals devoted to review articles. The first sample was compiled
in mid-2007 and consists of the 100 most cited papers in astronomy and astrophysics for 2001 to
2006 inclusive. For brevity I will refer to this sample as the “six-100” sample. Data for the
second sample – the one on which most of this paper is based - were assembled in early 2010
and consist of the 100 most cited papers in astronomy and astrophysics for each year in the past
decade 2000 – 2009. This sample will be referred to as the “ten-100” sample or, simply, “the
sample”. A full description of how papers in the six-100 and ten-100 samples were selected is in
Section 3. A comparison of citation counts from ISI and ADS is in Appendix A.
I will examine a number of statistics that characterize the papers in the two samples such
as their number of authors and citations, their page length, and where they were published.
Sections 3 through 6 will present the statistical analysis and the findings; Section 7 will
summarize the findings and suggest some interpretations, and identify areas worth further study.
While citation counting is not the only way to gauge a paper’s impact, it is a simple
number to extract from publication data bases and conceptually provides a straightforward way
of comparing papers. Abt (2000c) has in fact shown that “important papers almost invariably
produce many more citations than others, and citation counts are good measures of importance or
usefulness.” An obvious shortcoming is that citation counts are time dependent. Proposals to
minimize the impact of the “age” of a paper include the “h-index” and the “impact factor”
(Hirsch 2005; Bordons, Fernández, and Gomez 2002, but see Van Noorden 2010).
As an alternative to the absolute number of citations and to the h-index, I have chosen to
use the simple, non-parametric statistic of the ranking of a paper in terms of citation counts
relative to other refereed papers published in the same year. Thus a paper ranked “1” in its
sample for a year is the most cited in that sample as of the date that the data were compiled;
larger ranking numbers mean smaller number of citations. It turns out that this statistic can also
be time dependent, strongly so for the first few years immediately after publication. As will be
seen in section 3.1.4 an examination of this time dependency yields some interesting results as
well. Trimble and Ceja (2010) argue that the growth rate for citations to a paper can be strongly
subject dependent. Furthermore, Abt (1996, 1998) has shown that the “half lives” of papers vary
considerably. The results of the comparison of the ten-100 and six-100 samples in Section 3.1.4
probably reflect Trimble and Ceja’s and Abt’s findings. Nonetheless, rank has the advantage of
minimizing the effects of outliers and of giving an overall picture of the influence of a wide
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range of papers. For these same reasons, I will also use a ranking statistic to investigate other
parameters; again a “1” ranking will mean, for example, the largest number of authors or longest
paper, etc. in the group of 100 for a particular year.
Finally, there is the issue of self-citations. Trimble (1986) found that “About 15% of all
citations in astronomical papers published during January 1983 were self-citations, in the sense
that the cited and citing papers had at least one author in common.” She found that this
percentage varied little with parameters such as journal, country, topic, etc. Based on her study,
Abt (2000b) concluded that, although self-citations make up a non-negligible fraction of all
references, they do not, on average, distort citation statistics significantly. However, as this
paper will show, the average number of authors per paper for the top 100 has increased by more
than a factor of three over the past decade. This, together with the increasing prevalence of
papers with a large number (a dozen or more) of authors, probably means that the incidence of
self-citations has gone up considerably. Unfortunately, trying to correct for this effect is beyond
the scope of this paper (or the time available to the author!). Thus no corrections will be made
for self-citations in the analysis which follows.
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3. SOURCES FOR THE DATA AND THE SELECTION OF
JOURNALS AND PAPERS
3.1. Selection of Journals and Extraction of the 100 Most Cited Articles
3.1.1. Selection of Journals
In order to identify the refereed journals that publish papers on astronomy and
astrophysics I used the Journal Citation Reports (JCR) on ISI’s Web of Science to determine
which journals account for the top ~95% of all citations for each year. This was done twice for
the two samples but separated in time by about two years. I selected the category “Astronomy &
Astrophysics” on the JCR web page. There were 34 such journals in 2000 and 43 in 2008.1 For
each year I rank ordered the journals by total number of citations to articles published in that
journal between the year of publication and late 2007 (for six-100) or early 2010 (for ten-100) –
the times when I assembled the two data sets. The rank ordering by total citation numbers of the
journals year to year is quite consistent. Typically, those that comprise the bottom half of the list
account for 3% or less of the total number of citations for that year. At the top, the same 5
journals consistently account for between 80 and 85% of the citations in any given year. These 5
are the ApJ, ApJS, A&A, AJ, and MNRAS. Wanting to err on the side of inclusiveness I selected
those journals that together account for ~95% of the total citations to all journals in JCR’s
Astronomy and Astrophysics category in any given year.
After selecting the journals with 95% of the citations in a year, I sorted the remaining
ones by “impact factor” (as defined by JCR)2 and added to the initial list those that on average
had impact factors greater than three. This was to ensure that the study did not exclude small
journals with only a few highly cited papers. As a result, one journal was added to the list:
Astro-particle Physics. (Discussion of “issues” with journal impact factors are in Abt [2004,
2006a] and Van Noorden [2010]). In any case, no further use is made of impact factors in this
paper.) My final list had 19 journals. One of these made its first appearance in 2003 (“Journal of
Cosmology and Astroparticle Physics”) while another, A&A Supplements, merged with A&A in
2001. Thus, in this paper, data for A&A Supplements will always be merged with A&A itself.
Similarly, the numbers for the Astrophysical Journal include ApJ Letters (ApJL).3

1 Some of the journals listed in this category are not primarily astronomy related while other journals that
are primarily astronomy related are not included in the JCR list. Neither of these facts has a significant effect on the
analysis in this paper. Furthermore, neither Nature nor Science are on JCR’s list; they are categorized as “multi-
disciplinary”. These two weeklies required “special handling” as will be described later.
2 The JCR defines impact factor of a journal as follows: “[It] is calculated by dividing the number of
citations in the [JCR year] by the total number of articles published in the two previous years. An Impact Factor of
1.0 means that, on average, the articles published one or two year ago have been cited one time. An Impact Factor of
2.5 means that, on average, the articles published one or two year ago have been cited two and a half times.”
3 Until late 2007 ApJL was not identified separately from ApJ itself; so that all JCR searches relevant to
ApJ automatically included ApJL. Web of Science searches also did not distinguish between the main Journal and
the Letters except for the fact that page numbers for articles appearing in ApJL had an “L” before the number.
However, about the time that the AAS changed publishers for the ApJ the Web of Science and the JCR began to
treat the main journal and the letters as two separate journals. This, according to Thomson Reuters Customer
Technical Support is because the new publisher, IoP, assigned different ISSN numbers to the two. So when
Thompson Reuters receives the data for the journal from IoP it comes in as two separate journals and is entered as
such in their data base. So a Web of Science user must request information for both as distinct journals.
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Table 1 lists 17 of the 19 journals from which the top 100 articles are selected along with
summary statistics from ISI for 2000 – 2008; Nature and Science are not listed since they have to
be treated differently – see below. There are no entries for 2009 because JCR had not yet
compiled the statistics for that year when I did the citation search. Also, the ISI’s JCR online
database is missing the integrated data for Advances in Space Research for 2006. For each year
the two columns give the number of citations to all articles published in that journal for that year
(as of early 2010) and the number of those articles. The sum over all journals in Table 1 for each
year is a pretty constant fraction of the total number of articles published by all journals listed by
JCR in the category “astronomy and astrophysics”, namely between 84 and 89% except for 2000
when it goes up to 92%. Not surprisingly, the journals in Table 1include most of those in Abt’s
(2006) list.
Nature and Science are classified as “multi-disciplinary” by ISI rather than in the
astronomy and astrophysics category so they required special handling. I used the “General
Search” on ISI’s Web of Science pages and went year by year for each of the two journals, and a
list of topics that I determined with some trial and error. 4 As an additional aid in minimizing the
number of “spurious” results, I further restricted the search to specific types of “documents”, viz.
articles, database review, letters (but not research letters in the sense that Nature uses the term),
notes, and reviews. Once this search was done I then inspected each list to toss out non-
astronomical articles, book reviews, etc that made it through the filters.
Two of the journals in my final list are devoted to review articles, rather than articles
presenting new findings – Annual Review of Astronomy & Astrophysics, and Space Science
Reviews. These have been retained for this study since this is a survey of all the most influential
articles.
3.1.2. Selecting the 100 Most Cited Papers
The next step was to identify the “greatest hits”, the 100 most cited articles for each year.
Since the ApJ always has the lion’s share of citations, I used the Cited Reference Search on ISI’s
Web of Science to first select the 100 most cited papers for each year from the ApJ. This served
as an absolute lower limit for the selection of the 100 most cited from all journals. Then for each
of the other journals in Table 1 and for each year I selected those papers that had at least as many
citations as the paper that ranked 100 on the list for the ApJ. In spite of the fact that the journals
in Table 1 have already been selected from the full list of journals, nearly half of them never had
any papers with as many citations as the 100th ranked paper on the ApJ list.
Table 2 shows the distribution amongst the journals for the top 100 most cited papers in
the sample. The last line, “Other”, is for journals that generally had an average of 1 or less top-

Unfortunately, JCR does not input data for a “new” journal until there is a few years worth of data. Thus any data
extracted from JCR for the ApJ after late 2007 applies only to the main journal. Thus, for 2007, JCR shows 2796
articles in the main journal; while the Web of Science lists 52 additional articles in ApJL. For 2008 the
corresponding numbers are 2128 and 677, respectively. For 2009 they are 2795 and 697, respectively.The present
paper continues to treat the two parts of the ApJ as one and refers to it simply as “the ApJ”.
4 Here is the full string in case a reader wishes to do her or his own search: This string of topics with
Boolean operators seemed to be the most effective at both selecting all astronomy related articles while at the same
time minimizing the number of non-astronomical articles: "black hole" or astro* or cosmo* or solar* or planet* or
stellar* or star or galaxy or galactic or GRB or cosmic or "gamma-ray" or pulsar or Mars or Saturn or Pluto or
Titan or Uranus or Neptune or "deep impact" not climate not genetic not neuron* not human”.
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cited articles per year. This latter group includes Icarus and PASJ. An exception is JCosAPP.
Although it had zero or one articles that made the top 100 from 2003 (its first year of
publication) through 2008, in 2009 it had 10 articles. This table shows that only 6 journals
account for 85% of the 1000 most cited papers over the ten year period. Appendix B gives the
1st, 50th, and 100th ranked papers for each year.
The numbers in Table 2, though, do not tell the whole story though since the ApJ, A&A,
and MNRAS account for about 75% of all articles published in astronomy over the 10 year time
period based on the data from ISI’s JCR. Table 3 gives the percentages of articles from each
journal in Table 2 that make it into the top 100 for that year. For Nature and Science these
numbers refer to just how many articles on astronomy and astrophysics were identified for each
year as described previously. As may be seen, the most “successful” publication in terms of
fraction of articles published that make it into the top 100 is ARAA with Nature second and
Science third. Both Nature and Science, though, limit the number of astronomy articles that
appear in their weekly issues; thus they are already preselecting candidates for the top articles.
For comparison, three-fourths or more of the articles submitted for publication to the ApJ
(including Letters), AJ, A&A, and MNRAS eventually appear in those journals.5 For reference,
the average over 10 years of all articles published in these journals that make it into the top 100
is 1.4%. This indicates that on average A&A and MNRAS are relative under performers while AJ
and ApJ are about average. Low numbers for Solar Physics may in part reflect the relatively
small size of the solar community. The same is probably true for Icarus. Bear in mind, though,
that most of the small percentages have significant standard deviations (last column of Table 3).
With the procedure I followed there is still the possibility that some of the journals not
searched, i.e. not in Table 1, have articles with a high enough citation count to place them in the
top 100 for the year. To investigate this possibility, I went back to check if a sampling of the
non-Table 1 journals had any articles that would fall in the already selected top 100. Following a
suggestion by V. Trimble, I searched Acta Astronomica, Astronomy Reports, Astronomy Letters,
Astronomische Nachrichten, Journal of Astrophysics, Revista Mexicana Astronomy &
Astrophysics, Observatory, and Baltic Astronomy. Very few of these journals had any articles
with citation counts that came even within a factor of two of the last ranked article in my top 100
lists for each year.
However, 4 articles were found over the 10 year period that had more citations than the
100th ranked one for that year after the selection was made. There were two in Acta Astronomica
in 2002 with citation counts of 210 and 192. These numbers would have placed both in the
lower half of the top 100 for that year. Both presented results of large-scale surveys, one an all-
sky survey for variable stars, the second the results of the 2001 campaign of OGLE, the Optical
Gravitational Lensing Experiment. Most of the same issue of Acta Astronomica contained
results from OGLE. The other two articles that would have made it into the top 100 are both
from Astronomische Nachrichten and both have to do with technical aspects of the Sloan Digital
Sky Survey (SDSS). They were from 2004 and 2006 with 250 and 158 citations, respectively; so
would rank about 30th in the highly cited list for their years. All four of these articles were found
after the analysis presented in this paper was completed. Since they would constitute only 0.4%
of the 1000 articles that are the basis of the analysis, I did not redo the analysis to include them.

5 In the case of the ApJL, about 60% are accepted into the Letters while another 15% get moved to the main
journal.
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3.1.3. Further Comments on Table 2 and Table 3
As pointed out earlier, since Nature and Science are multi-disciplinary journals, I needed
to use the ISI search engines to select, based on keywords, a preliminary list of articles that likely
dealt with astronomical topics for each year. I then went through these lists usually by title but
occasionally with the help of the abstract, to excise any remaining non-astronomical articles.
Thus the total count per year for these two journals may be biased in undercounting astronomy
related articles. With these caveats in mind, for 2007 – 2009 these journals each published about
50 astronomy related articles, or one a week on average. In 2005 and 2006 Nature had 90 and 72
articles, respectively, while Science had 67 and 63, respectively. For Nature these two years were
also ones with more than 20% of its astronomy papers making it into the top 100 (see Table 3).
For both journals these numbers dropped down into the 30s for 2000 to 2002. For 2005 and
2006 both journals had a considerable number of papers presenting results from the Cassini
mission to Saturn and Titan, the various missions to Mars, Voyager 1, and Deep Impact. Nature
had several Voyager 2 papers in the top 100 in 2008. For 2000 the three Science papers that
made it into the top 100 were all from the Mars Global Surveyor.
The ApJS has a marked upward spike in 2003. This was previously pointed out by Abt
(2006a; see also Trimble and Ceja 2008) who attributed it to “the extremely high citation rates …
for 13 papers in the special issue devoted to the Wilkinson Microwave Anisotropy Probe”.
The small up tick for PASP in 2003 is due to a series of articles that give overviews of the
Spitzer legacy projects.
As a partial check on long term trends I carried out identical searches for the top 100
articles for 1990 and 1995. The last two columns of Table 2 show the distribution amongst
journals for these years. Within the scatter there is not much change going from these two years
to the decade sample, although several journals may show a decline in their contribution to the
top 100 while a couple of others may have gone up a bit. Two journals that now make small
contributions to the top 100 but did not even exist in the 1990s are JCosParAp and Astropart.
Phys.
3.2. The Time Evolution of Citation Rankings Based on a Comparison of Two Samples
Since the six-100 and ten-100 samples were assembled about two and a half years apart, a
comparison of overlapping years should indicate something about the time evolution of the
rankings of articles by citation numbers. A related examination of the lifetimes of astronomical
papers was made by Abt (1998) and Abt and Boonyarak (2004).
Data for the six-100 sample were assembled in late 2007 while data for the ten-100
sample were assembled in early 2010. The second column of Table 4 lists the number of papers
in the top 100 samples for the years given in column one that were in the earlier sample but did
not make it into the later one, having been replaced by other papers. The third and fourth
columns give the average rankings of the papers that were dropped (Six-100 sample) and those
that replaced them (Ten-100 Sample). This table shows that the number of non-overlap papers
declines quickly with age. Also that the papers that dropped out of the six-100 sample are
generally ranked in the bottom quarter of the sample, while the new ones that came in, although
still in the bottom half of their ten-100 samples, have, on average, systematically higher rankings
than those that they replaced. These results are not surprising. As papers age, their citation
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ranking will generally stabilize. And it should be much easier to dislodge a paper from near the
bottom of the pile than one in the top half.
Are there any trends apparent in the samples of non-overlap papers? First, I broke them
down by journal – noting those journals that lost papers from the six-100 sample and those that
gained papers in the newer, ten-100 sample. These results are given in Table 5 where a negative
entry indicates a loss of articles. The last two columns give the arithmetic sum for each journal
and its standard deviation. Based on these numbers there appear to be some statistically
significant winners and losers. The ApJ and MNRAS are the losers while the winners are A&A,
ARAA, and PASP.
There is an interesting scientific thread in the lost and gained samples. Examination of
the article titles revealed a strong bias as to topic. For 2001 to 2003, while about 11 of the 13 or
14 papers lost each year (see Table 4) covered pretty much the entire electromagnetic spectrum,
they all dealt with extra-galactic topics while only three to five of the new papers that replaced
them were extra-galactic. For 2004 amongst the “lost” papers were 11 extra-galactic ones and 5
from the Martian Rovers while the 2004 “new” papers had 8 extra-galactic ones and only one
about Mars. For 2005, the “lost” and “new” samples had 20 and 10 extra-galactic papers,
respectively. Finally, for 2006, of the 42 lost papers, 33 were extra-galactic to be replaced by 32
other extra-galactic ones.
Overall this suggests that extra-galactic papers have a more rapid rise, about one to three
years, in their citation counts than papers on other research areas. After this initial rapid rise, the
rate at which extra-galactic papers accumulate citations drops down to or below that of the
papers in the other areas. Furthermore, the lost papers appear to be dominated by observational
studies whereas the new ones are more heavily weighted towards theory and instrumentation.
These findings are consistent with Trimble and Ceja’s (2008, 2010) that the rate at which
citations accumulate can be a strong function of a number of different variables, particularly the
field of research, and Abt’s (1996, 1998) observation that the half lives of papers can vary
considerably. Trimble and Ceja also note that overall the rate at which citations accumulate,
especially in the first few years after a paper’s publication, is much steeper than linear. All of
these issues will be considered more fully in a subsequent paper.