The impact factor
, often abbreviated
, is a measure reflecting the average number of
to articles published in science and social science journals
is frequently used as a proxy
the relative importance of a journal within its field, with
journals with higher impact factors deemed to be more important
than those with lower ones. The impact factor was devised by
, the founder of the
(ISI), now part of Thomson Reuters
. Impact factors are
calculated yearly for those journals that are indexed in Thomson
Reuter's Journal Citation
In a given year, the impact factor of a journal is the average
number of citations to those papers that were published during the
two preceding years. For example, the 2003 impact factor of a
journal would be calculated as follows:
- A = the number of times articles published in 2001 and
2002 were cited by indexed journals during 2003
- B = the total number of "citable items" published in
2001 and 2002. ("Citable items" are usually articles, reviews,
proceedings, or notes; not editorials or
- 2003 impact factor = A/B
(Note that 2003 impact factors are actually published in 2004; it
cannot be calculated until all of the 2003 publications had been
received by the indexing agency.)
New journals, which are indexed from their first published issue,
will receive an impact factor after two years of indexing; in this
case, the citations to the year prior to Volume 1, and the number
of articles published in the year prior to Volume 1 are known zero
values. Journals that are indexed starting with a volume other than
the first volume will not get an impact factor until they have been
indexed for three years. Annuals and other irregular publications,
will sometimes publish no items in a particular year, affecting the
count. The impact factor relates to a specific time period; it is
possible to calculate it for any desired period and the Journal
(JCR) also includes a 5-year impact factor.
The JCR shows rankings of journals by impact factor, if desired by
discipline, such as organic
The IF is used to compare different journals within a certain
field. The Web of Knowledge indexes 9000 science and social science
journals from 60 countries and the results are widely (though not
freely) available. In addition, the IF is an objective
Numerous criticisms have been made of the use of an impact factor.
Besides the more general debate on the usefulness of citation
metrics, criticisms mainly concern the validity of the impact
factor, possible manipulation, and its misuse.
- The impact factor could not be reproduced in an independent
audit (but see Thomson Scientific's reply).
- The impact factor refers to the average number of citations per
paper, but this is not a normal
distribution. It is rather a Bradford
distribution, as predicted by theory. Being an arithmetic mean, the impact factor therefore
is not a valid representation of this distribution and unfit for
- In the short term - especially in the case of low-impact-factor
journals - many of the citations to a certain article are made in
papers written by the author(s) of the original article. This means
that counting citations may be independent of the real “impact” of
the work among investigators. Garfield, however, maintains that
this phenomenon hardly influences a journal's impact factor.
However, a study of author self-citations in diabetes literature
found that the frequency of author self-citation was not associated
with the quality of publications. Similarly, journal self-citation
is common in journals dealing in specialized topics having high
overlap in readership and authors, and is not necessarily a sign of
low quality or manipulation.
A journal can adopt editorial policies that increase its impact
factor. These editorial policies may not solely involve improving
the quality of published scientific work.
- Journals may publish a larger percentage of review articles
which generally are cited more than research reports. Therefore
review articles can raise the impact factor of the journal and
review journals will therefore often have the highest impact
factors in their respective fields.
- Journals may change the fraction of "citable items" compared to
front-matter in the denominator of the IF equation. Which types of
articles are considered "citable" is largely a matter of
negotiation between journals and Thomson Scientific. As a result of
such negotiations, impact factor variations of more than 300% have
been observed. For instance, editorials in a journal are not
considered to be citable items and therefore do not enter into the
denominator of the impact factor. However, citations to such items
will still enter into the numerator, thereby inflating the impact
factor. In addition, if such items cite other articles (often even
from the same journal), those citations will be counted
and will increase the citation count for the cited journal. This
effect is hard to evaluate, for the distinction between editorial
comment and short original articles is not always obvious. "Letters
to the editor" might refer to either class.
- Several methods, not necessarily with nefarious intent, exist
for a journal to cite articles in the same journal which will
increase the journal's impact factor.
- In 2007 a specialist journal with an impact factor of 0.66
published an editorial that cited all its articles from 2005 to
2006 in a protest against the absurd use of the impact factor. The
large number of citations meant that the impact factor for that
journal increased to 1.44. As a result of the increase, the journal
was not included in the 2008 Journal Citation Report.
- The impact factor is often misused to evaluate the importance
of an individual publication or evaluate an individual researcher.
This does not work well since a small number of publications are
cited much more than the majority - for example, about 90% of
Nature's 2004 impact factor was based on only a quarter of its
publications, and thus the importance of any one publication will
be different from, and in most cases less than, the overall number.
The impact factor, however, averages over all articles and thus
underestimates the citations of the most cited articles while
exaggerating the number of citations of the majority of articles.
Consequently, the Higher Education
Funding Council for England was urged by the House of
Commons Science and Technology
Select Committee to remind Research Assessment Exercise
panels that they are obliged to assess the quality of the content
of individual articles, not the reputation of the journal in which
they are published.
Other measures of impact
Some related values, also calculated and published by the same
- the immediacy index: the number
of citations the articles in a journal receive in a given year
divided by the number of articles published.
- the cited half-life: the median age of the
articles that were cited in Journal Citation Reports each
year. For example, if a journal's half-life in 2005 is 5, that
means the citations from 2001-2005 are half of all the citations
from that journal in 2005, and the other half of the citations
- the aggregate impact factor for a subject
category: it is calculated taking into account the number of
citations to all journals in the subject category and the number of
articles from all the journals in the subject category.
These measures apply only to journals, not individual articles or
individual scientists (unlike the H-index
The relative number of citations an individual article receives is
better viewed as citation
It is, however, possible to measure the Impact factor of the
journals in which a particular person has published articles. This
use is widespread, but controversial. Garfield warns about the
"misuse in evaluating individuals" because there is "a wide
variation from article to article within a single journal". Impact
factors have a large, but controversial, influence on the way
published scientific research is perceived and evaluated.
In 1976 a recursive impact factor that gives citations from
journals with high impact greater weight than citations from
low-impact journals was proposed.Such a recursive impact factor
resembles the PageRank
algorithm of the
search engine, though the original
Pinski and Narin paper uses a "trade balance" approach in which
journals score highest when they are often cited but rarely cite
other journals. A number of subsequent authors have proposed
related approaches to ranking scholarly journals.In 2006, Johan
Bollen, Marko A. Rodriguez, and Herbert Van de Sompel
using the PageRank
algorithm. From their
The table shows the top 10 journals by ISI
PageRank, and a modified system that combines the two (based on
2003 data). Nature
regarded as the most prestigious journals, and in the combined
system they come out on top.
is another PageRank
-type measure of journal influence, with
rankings freely available online.
- H-index, for the impact factor of
individual scientists, rather than journals.
- PageRank, the algorithm used by
Google, based on similar principles.
- Eigenfactor, another journal
citation ranking method.
- SCImago Journal Rank, an
open access journal metric which is based on Scopus data and uses
an algorithm similar to PageRank.
- S.A. Marashi. On the identity of “citers”: are papers promptly
recognized by other investigators? (2005) Med. Hypotheses 65, 822.
- Impact Factor, Immediacy Index, Cited