With the ready availability of scientific articles online, many of which are open
access and do not require subscription or pay‐per‐view, current information about
science and medicine has become easier to access than ever before. In the past 14
years, the number of new articles that appeared in PubMed more than doubled from 593,740
in 2003 to 1,255,875 in 2016. This explosion of information and the revolution in
how this information is distributed have made it more challenging than ever for scientists
and clinicians to keep up with research activity in their areas of endeavor. Investigators
must find ways to filter all of this information and find the highest quality and
most relevant articles and journals for the limited amount of time they have to read
the literature. Currently, readers can evaluate the available scientific literature
using three different methods: citation metrics, usage metrics, and alternative metrics
(so‐called altmetrics).
One of the most time‐honored quality indicators of the scientific literature is the
impact factor, a citation metric first proposed by linguist Eugene Garfield in 1955
and developed in the 1960s to compare the quality of one journal to another in a given
field. Thus, impact factor is a journal level as opposed to an article level metric.
It is calculated as the number of citations in the literature of the current year
(census year) to papers published in a journal in the preceding 2 years (the target
period) divided by the number of citable items published in the journal during those
2 years. For example, if a journal published 100 articles in the time period 2014–2015
and 150 citations were made to those articles in 2016, the journal's 2016 impact factor
would be 1.5. The usefulness of the impact factor depends on the accuracy of the citation
counts used in its calculation. The citation data used to calculate impact factor
are derived from the Web of Science database, a subscription‐based scientific citation
indexing service operated by Clarivate Analytics. The 2015 impact factor of the Journal
of Veterinary Internal Medicine was 1.821, and the Journal ranked 19th of 138 journals
in the Veterinary Sciences category of Journal Citation Reports.
For many years, impact factor has been the “gold standard” for assessing quality in
the scientific literature, and it has been used in many ways, some of which likely
were not intended when it was first developed and for which it is not well suited.
It has been used by scientists and clinicians to determine which journals they read
and where they submit their work, and by academic administrators to assess the quality
of the research of faculty members as well as their funding potential and suitability
for promotion and tenure.
Since the 1980s, however, the supremacy of the impact factor has been called into
question for various reasons. One major concern is that impact factor is a lagging
indicator. Citations to published articles accrue slowly. For example, it may take
a year from submission of a manuscript until its publication in a traditional print
journal and another 1–2 years before citations to the article start to appear in the
literature. Such a time frame simply is not fast enough in today's internet‐driven
world. Furthermore, impact factor is not a direct measure of quality. Journals in
different disciplines and even within a given discipline cannot necessarily be compared
to one another. For example, rate of publication typically is lower in the humanities
as compared to the sciences, and niche journals in a given discipline typically are
cited less frequently than are general journals. Less frequent publication of articles
by authors in some fields and less frequent citation of niche journals can adversely
affect impact factor regardless of journal quality. Impact factors are subject to
gaming by authors, editors, and publishers. A journal that publishes large numbers
of review articles may receive a higher impact factor than one that publishes only
original research because review articles tend to be heavily cited. Self‐citation
by authors and encouragement by journal editors for authors to cite other papers previously
published in their journals also can affect impact factor. Citation stacking is another
method of gaming that involves reciprocal citation between colluding journals in an
attempt to boost the impact factors of both journals without resorting to self‐citation.
Other journal level metrics calculated in Journal Citation Reports include immediacy
index, eigenfactor, and article influence score. The immediacy index is the average
number of times an article was cited in the year it was published and reflects how
quickly articles appearing in a given journal are cited in the literature. The eigenfactor
score was developed by Jevin West and Carl Bergstrom and is an indicator of the importance
of a given journal to the scientific community. Journals are rated according to number
of citations received, but citations are weighted such that citations from more highly
ranked journals contribute more than do citations from lower ranked journals. The
eigenfactor score is influenced by the size (i.e., number of articles published per
year) of the journal such that it doubles when journal size doubles. The article influence
score is a reflection of the average influence of a given journal's articles over
the first 5 years after their publication. It is derived from the eigenfactor score
and is a ratio of the journal's citation influence to the size of the journal's article
contribution over a 5‐year period.
Google Scholar is a free citation index operated by Google. It covers not only journals
but books, theses, and other items deemed to be academic in nature. Several journal
level metrics are provided by Google Scholar, including the H5 Index, a variation
on the h‐index. The h‐index was proposed by Jorge Hirsch in 2005 as a means to determine
the scientific productivity and impact of individual scientists, but its use has been
extended to groups of scientists, as well as to individual articles and journals.
It represents an attempt to assess impact by measuring productivity (number of published
articles) and citations to these published articles. The h‐index is defined by how
many papers, h, have at least h citations each. The h‐index favors authors, articles
or journals that have been in the literature longer because it counts all citations
without weighting them by age.
Scopus is a citation index owned and operated by Elsevier and available by subscription.
Scopus covers approximately 22,000 journal titles as compared to approximately 12,000
journals covered by its main competitor, the Web of Science. Journal metrics derived
from the Scopus database include SNIP (Source Normalized Impact per Publication) and
SJR (SCImago Journal Rank). Source Normalized Impact per Publication normalizes citation
count based on the citation potential of a given subject area. Thus, it allows comparison
of journals in different subject areas with different levels of citation activity.
The value of such a metric for use within a single field such as veterinary medicine
is uncertain. The SJR is another journal level metric derived from Scopus data that
weights the citations that a journal receives based on the quality of the journal
in which these citations appear, and in this way it is similar to the eigenfactor
and article influence scores. Recently, Elsevier announced another journal metric
called CiteScore derived from the Scopus database. It is similar to impact factor
but covers a 3‐year citation window and includes not only articles and review papers
but letters, notes, editorials, conference papers and other documents indexed by Scopus.
The Journal of Veterinary Internal Medicine had SNIP and SJR scores of 1.194 and 1.257,
respectively, in 2015, and a CiteScore of 2.09 (ranking it 11th among 150 veterinary
journals).
Although accepted indicators of journal quality, none of the metrics described above
provide information about the usefulness of a given article to individual readers.
A corollary of the impact factor is the citation rate of individual articles. Implicit
in the practice of monitoring and reporting citation rates is the assumption that
number of citations received is related to the utility of an article. Some articles
may have markedly influenced clinical practice yet not be cited frequently in the
literature. However, impact factor cannot take into account the sentiment of a citation.
For example, a previously published article may be heavily cited not because the research
was brilliant and highly influential in the field, but because it was wrong and delayed
progress (i.e., refutation could have been the reason for citation). Self‐citation
as a consequence of personal vanity and gift citations to well‐respected authors for
political reasons are other practices that can affect the number of citations of an
individual article. The clinical value of articles might be better assessed by alternative
metrics that provide information about article usage (e.g., downloads) or sharing
of articles among readers (e.g., posts and shares on social media). Such indicators
might favor articles published in open access journals as compared to those that require
a paid subscription or pay‐per‐view access.
Thanks to the increased availability of journals online in recent years (either as
paid subscriptions or open access), usage metrics have emerged as a relatively new
way to assess the impact of articles published in the scientific literature. The primary
usage metrics are page views (including numbers of unique users and time spent at
a given site) and full‐text article downloads. For example, the Journal of Veterinary
Internal Medicine had 2,047,525 page views in 2016 with 968,183 (47%) coming from
the United States and United Kingdom. An average of 1,066 accesses per article occurred
in 2016 for content published in 2016 as compared to an average of 788 accesses per
article in 2015 for content published in 2015 (a 35% increase). The Journal had 1,351,225
article downloads in 2016 by more than 396,000 unique visitors as compared to 1,071,766
downloads in 2015 (a 26% increase). The primary advantage of usage metrics is that
data begin to accrue immediately after publication and can be readily collected and
reliably analyzed. Another advantage of usage metrics is that they allow use of articles
by lay as well as scientific audiences to be assessed, provided online publication
is open access (as is the case for the Journal of Veterinary Internal Medicine).
However, usage metrics also have disadvantages and potentially might be misleading.
For example, individuals can generate large numbers of page views while browsing the
literature online but without actually reading the articles. Also, it is important
to remember that the number of full‐text downloads of articles cannot be equated with
the number of articles actually read and put to use in clinical or scientific work.
Clearly, individuals can download the full text of recently published articles without
ever reading the articles themselves. The same however can be said of citations in
that authors can cite articles without having actually read the article or perhaps
having only read the abstract. Likewise, downloading articles to reference management
software (e.g., Endnote, RefWorks, Reference Manager) with intent to read the articles
later does not necessarily mean the articles will ever be read or used. Kurtz and
Bollen have said, “… there is no clear consensus on the nature of the phenomenon that
is measured by download counts” (Ann Rev of Info Sci and Tech 44:3‐64, 2010). Furthermore,
like impact factor, article downloads can be gamed using download bots and other strategies
to inflate the number of downloads. Finally, the usefulness of downloads as a predictor
of future citations is as of yet unknown. There may be a novelty effect in which large
numbers of downloads occur soon after publication but do not translate into citations
later.
Alternative metrics (or “altmetrics”) are article level metrics aimed at quantifying
how scientists and the general public find, share and discuss articles in literature
using nontraditional lines of communication. Examples of such interactions include
Facebook posts, Reddit posts, Tweets, blog posts, social bookmarking, social media
sharing, media mentions, and Wikipedia citations. One of the primary features of “altmetrics”
is their immediacy. They begin to accumulate as soon as an article is published online
and spread widely among users of social media. Alternative metrics have gained traction
as a supplemental measure of scholarship quality because the new generation of scholars
has embraced social media as a way to discover and share research. “Altmetrics” close
the gap between article publication and citation.
The reasons people post and share information about published articles however are
not always clear and potentially are unknowable. As Stacey Konkiel (who joined Altmetric
in 2015 as Director of Research and Education) said in the July/August 2013 issue
of Information Today's Online Searcher, “the viral nature of the web can lead to extremes
in altmetrics counts, which have led some to make the distinction between two types
of research: ‘scholarly’ and ‘sexy.’” And, how can one differentiate the “scholarly”
versus “sexy” attributes that have resulted in a published article accumulating large
numbers of Tweets and Facebook posts? Alternative metrics eventually may allow assessment
of context (i.e., determining why a paper is being used) through “text mining” (identifying
when an article is mentioned but not linked to). In the meanwhile, interpretation
of “altmetrics” requires careful scrutiny on the part of the end user, which requires
an investment of time that many scientists and clinicians simply do not have. Finally,
like usage metrics, alternative metrics can be gamed, for example, by purchasing Tweets,
Facebook posts, and blog mentions.
The digital science company Altmetric, based in London, was founded by Euan Adie in
2011. It quantifies alternative metrics such as news stories, blog posts, Tweets,
Facebook posts, peer reviews, Reddit posts, F1000 articles (the “Faculty of 1,000”
is an international group of scientists and clinicians who rate and recommend articles
in biology and medicine), and Wikipedia citations and determines an “Altmetric Score”.
The #1 article in 2016, with a score of 8,063, was a special communication about progress
in healthcare reform published in the Journal of the American Medical Association
by Barack Obama. One can see how this publication landed where it did on the list.
Lower on the list, at #43 and with an Altmetric score of 2,078, was a paper published
in Proceedings of the National Academy of Sciences about a 5,000‐year‐old beer recipe
from China. It's less clear why this paper landed on the list – perhaps it was great
science, perhaps people found it amusing, or both. Examples of other companies that
provide tools to track alternative metrics include Impactstory and Plum Analytics.
The Altmetric score and types of usage for an article published in the Journal of
Veterinary Internal Medicine in 2016 are shown in Figure 1. This article has been
downloaded over 12,000 times since publication, and its Altmetric Score places it
in the 99th percentile of all articles monitored by Altmetrics.
Figure 1
An example of an article published in the Journal in 2016 with an Altmetrics Score
of 704 showing media coverage, blog mentions, Tweets, Facebook mentions, Google+ posts,
and readers on Mendeley.
Ultimately, each reader must decide which articles to read and use in his or her own
area of clinical or scientific endeavor. Different metrics to assess the value and
impact of contributions to the scientific literature have evolved in tandem with changes
in how information is distributed to readers – from traditional print publication
of articles to online publication and open access. Each method of assessment has its
own merits and vulnerabilities. Readers will be best served using all three tools
– citation, usage and alternative metrics – together in a complementary fashion, and
the editors of the Journal will continue to monitor all three types of indicators
to assure that we continue to publish high‐quality content that meets the needs of
our readers.