Impact factor

The arithmetic is a simple ratio: citations this year to items published in the previous two years, divided by the number of those citable items. Because citation counts are heavily skewed, a handful of highly cited papers can lift a journal's figure while most of its articles are cited rarely, so the average poorly represents any single paper.

The metric is also gameable and field-dependent. Editorial choices about review articles, self-citation, and what counts as a citable item all move the number, and fields with fast, dense citation practices score higher than slower ones regardless of quality. The San Francisco Declaration on Research Assessment (DORA) urges institutions to stop using it to judge individual work.

For evaluating a specific manuscript, the impact factor of its target journal says essentially nothing about the paper's validity. Reviewers and authors are better served by assessing the study itself than by the prestige of where it lands.