The impact factor, properly the Journal Impact Factor (JIF), is a bibliometric measure of how often, on average, articles in a journal are cited within a defined recent period. It is published annually by Clarivate in the Journal Citation Reports (JCR) and is one of the most influential, and most contested, numbers in scholarly publishing. Understanding what it actually measures is the first step to using it responsibly.
In short: the impact factor is a property of a journal, calculated from citation averages, and it was never designed to evaluate individual articles or researchers.
How the impact factor is calculated
The standard two-year Journal Impact Factor for a given year is a ratio. The numerator is the number of citations received in that year to items the journal published in the two preceding years. The denominator is the number of citable items, typically research articles and reviews, published in those same two years.
| Component | What it counts |
|---|---|
| Numerator | Citations in the current year to articles from the previous two years |
| Denominator | Number of citable articles published in those two years |
| Result | Average citations per citable article over the two-year window |
A journal with a JIF of 5 received, on average, five citations per citable article published in the preceding two years. Clarivate also publishes a five-year variant that widens the citation window for fields where impact accrues more slowly.
What the impact factor does and does not measure
The JIF captures the recent average citation rate of a journal’s body of work. It can offer a rough sense of how actively a journal’s content is being cited within a couple of years of publication. What it does not measure is the quality, rigour or importance of any single article, because citations within a journal are highly skewed: a small number of heavily cited papers can pull the average up while most articles receive far fewer citations. The mean is therefore a poor predictor of any individual paper’s citations.
Other well-known limitations include differences in citation culture between disciplines, which make cross-field comparison misleading, and the inclusion or exclusion decisions about what counts as a citable item, which can affect the denominator.
Garfield’s own caveats
Eugene Garfield, who originated the impact factor as a tool for selecting journals for indexing, repeatedly cautioned against misusing it. He noted that the metric was intended to help librarians and editors compare journals, not to judge the worth of individual scientists, and warned that the skewed distribution of citations meant the journal average should not be read across to the papers it contains. The critique of the metric is therefore not an external attack so much as a return to its creator’s own warnings.
Why DORA and the Leiden Manifesto warn against misuse
Two landmark statements formalise these concerns. The San Francisco Declaration on Research Assessment (DORA) recommends not using journal-based metrics such as the JIF as a surrogate measure of the quality of individual research articles, or to assess an individual scientist’s contributions for hiring, promotion or funding decisions. The Leiden Manifesto for research metrics sets out ten principles for the responsible use of quantitative indicators, including that quantitative evaluation should support, not replace, expert qualitative assessment, and that metrics should be measured against the research mission rather than imposed generically.
These principles are central to the wider shift covered in our responsible assessment coverage, and they sit alongside author-level measures discussed in our standards dictionary. The common thread is simple: a journal-level average should never stand in for reading the work itself.
Using the impact factor sensibly
Responsible use means treating the JIF as one descriptive feature of a journal rather than a verdict on the papers within it or the people who wrote them. Where citation context matters, field-normalised indicators and a basket of complementary metrics are more defensible than a single number. Crucially, evaluation of individuals should rest on the content and contribution of their outputs, an approach reinforced across our guidance for authors.
Frequently asked questions
What citation window does the standard impact factor use?
The standard Journal Impact Factor uses a two-year window: it counts citations in the current year to articles the journal published in the previous two years, divided by the number of citable items in those two years.
Who calculates and publishes the impact factor?
The Journal Impact Factor is calculated and published annually by Clarivate in the Journal Citation Reports, based on its citation database.
Why is it wrong to judge a researcher by journal impact factors?
Because citations within any journal are highly skewed, the journal average does not predict an individual article’s citations. DORA explicitly recommends against using journal-based metrics as a proxy for the quality of individual articles or researchers.
What should be used instead?
Responsible assessment favours reading the work, supported where appropriate by article-level and field-normalised indicators and expert qualitative judgement, as set out in DORA and the Leiden Manifesto.
