Impact factor

The Impact Factor (IF) or Journal Impact Factor (JIF) is a citation-based metric that reflects the yearly average number of citations of articles published in the last two years in a given journal. It is frequently used as a proxy for the relative importance of a journal within its field; journals with higher impact factors are often deemed to be more important, or carry more intrinsic prestige in their respective fields, than those with lower impact factors.

Impact factors can be used as one of a range of measures when comparing journals. However impact factors do not take account of citation patterns across different fields and so you cannot use them to accurately compare journals across different disciplines. As a journal-level metric impact factors should not be used to evaluate the merit of individual articles or researchers. This aligns with recommendation one in the San Francisco Declaration on Research Assessment (DORA).

How is an impact factor calculated?

In any given year the two-year journal impact factor is the ratio between the number of citations received in that year for publications in that journal that were published in the two preceding years, and the total number of citable outputs published in that journal during the two preceding years. An example is provided below (source: Wikipedia:​

Impact Factor

Criticism of impact factors

Impact Factors are generally viewed in the scholarly community as fundamentally flawed in that they present the mean of data that are not normally distributed, and so they are influenced heavily by a small number of highly-cited papers in a journal. For example, about 90% of Nature's 2004 impact factor was based on only a quarter of its publications, and thus the actual number of citations for a single article in the journal is in most cases much lower than the impact factor would suggest. Impact Factors can also be easily gamed. Journals can adopt editorial policies such as limiting the number of citable outputs (thereby reducing the denominator in the equation), they can publish outputs expected to be highly cited early in the calendar year to allow maximum time for the papers to be cited, and some journals even ask authors to add extraneous citations to an article before the journal will accept it (a practice known as coercive citation).​