Definition

Impact factor

A journal metric: the mean citations in a year to items the journal published in the previous two years. A journal average — not a paper’s or author’s quality score.

The Journal Impact Factor was built for librarians comparing subscriptions, and as such it works. Its misuse — judging individual papers or researchers by their journal’s average — is statistically indefensible: citation distributions are so skewed that most papers sit far below their journal’s IF, which a few blockbusters drag upward.

Reform declarations (DORA, the Leiden Manifesto) commit institutions to assessing research on its own merits; alternative indicators (per-paper citations, field-normalized metrics, OpenAlex-style open data) keep improving. Useful heuristic: journal metrics for venue choice, article metrics for articles, reading for judgment.