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Impact factors
Impact factors help evaluate the relative importance of a journal based on how often scholars and researchers cite articles from that journal. The Impact Factor (IF) for a journal is calculated using data from the Web of Science databases (available via Web of Knowledge) using the formula shown below. The higher the impact factor the better and the more influential the journal can be considered. As a benchmark, a journal impact factor of 1.0 means that, on average, articles published in that journal one or two years ago have been cited once.The formula used to calculate a journal's impact factors is:
Numerator: number of
citations in a given year to articles
published in the journal from the
previous two years
Denominator: Number of articles
published in the journal in previous two
years.
The definitive source for impact
factors is the Journal Citation Reports (JCR)
database published by Thomson Reuters
Scientific. This lists journal
titles and assigns an impact factor to each. It covers over 7600 journal titles
from 3300 publishers worldwide and offers
a Science Edition and a Social Sciences
Editions. JCR can
help identify: the most frequently cited
journals in a field, the highest impact journals
in a field, and the most prominent journals in a
field. JCR is the only source of citation
data on journals, and includes virtually all
areas of science, technology, and social
sciences. It is available through
Web of Knowledge and a
factsheet and
training materials are available.
Find a quick-list of high impact
journals in various subject areas with the
Science Gateway
(this uses data from the Journal Citation
Reports).
Pros and cons
√ Impact factors gives researchers a quantitative measure of journals' influence and impact.
√ The impact factor is a simple metric and provides a consistent way of comparing journals.
√ Ranking is consistent within fields of study.
X Impact factors can be affected by issues unrelated to a journal's quality e.g. citation bias (authors citing
their own
work), journals self-citing, publication timing and types of
articles published.
X An impact factors is not a measure of individual author citation impact. It shouldn't be used to measure the
performance of an author. An alternative measure of researcher output is the H-Index or Hirsch Index.
X A large number of citations does not necessarily mean an article is of good quality. It may be cited heavily
because many authors are refuting its findings.
X Impact factors need two years of data meaning no data is available for newer journals. There is an an
alternative metric called the immediacy index. This is similar to the impact factor but is calculated on the
number of cites in the most recent year. The immediacy index of a journal is available in the Journal Citation
Reports (JRC) from Web of Knowledge.
There is also a measure called the R-Index or Reliability-Based-Citation Impact Factor which incorporates
data over the lifespan of a journal rather than the journal's recent performance history.
X Only journals included in Web of Science databases are included.
X Thomson Reuters Scientific, which updates impact factors every year, does not share their criteria for what
constitutes a "citable paper," which is part of the impact factor equation's denominator.
Related information
The Eigenfactor - a measure of the journal's total importance to the scientific community which ranks journals much as Google ranks websites, reports journal price, includes other materials as well as journals, adjusts for citation differences across disciplines based on 5 years of citation data. It aims to improve on the h-indexThe Citation Impact Center - a community forum offered by Thomson Reuters
MyRI: Measuring your Research Impact - an open access toolkit, providing an overview to bibliometrics; how journal impact factors are measured; and how to track research impacts at individual, departmental, institutional and country level.