Journal impact factors are calculated and reported annually by Clarivate Analytics' Journal Citation Reports (WSU subscription)

Journals with higher impact factors publish articles that are cited a greater number of times, on average, than journals with lower impact factors. For example, if a journal has a 2018 impact factor of 4, this means that articles published in 2016 and 2017 were cited 4 times, on average, in 2018.

Impact factors should not be compared across different disciplines. Different disciplines vary in their publishing and citation practices, which can influence the magnitude of journal impact factors. Journals in the life sciences tend to have the highest impact factors, and basic science journals tend to have higher impact factors than clinical medicine journals.

Impact factors should not be used to evaluate individual articles. In any given year, a small proportion of articles (~30%) published in a journal account for most (~70%) of that journal's citations (source); therefore, publication in a high impact journal does not mean that the article is frequently cited. Also, the scientific quality of a study does not always correlate with journal ranking. >

Impact factors should not be used to evaluate researchers. It is common for hiring panels, T&P committees, and granting agencies to evaluate researchers based on the impact factors of the journals in which they publish. This practice of rewarding researchers based on publishing in high impact journals can skew scientific progress by encouraging scientists to restrict their research to topics that are more likely to be published in top journals.

P.O. Seglen. (1997). Why the impact factor of journals should not be used for evaluating research. BMJ, 314(7079): 498-502.

E. Garfield. (2006). The history and meaning of the journal impact factor. JAMA, 295(1): 90-93.

S. Saha, S. Saint, D.A. Christakis. (2003). Impact factor: A valid measure of journal quality? JMLA, 91(1): 42-46.

R. Monastersky. (2005). The number that's devouring science. The Chronicle of Higher Education. 

A. Casadevall, F.C. Fang. (2014). Causes for the persistence of impact factor mania. mBio, 5(2): e00064-14.

In addition to the original impact factor, which measures the average impact of articles over a ~2-year time span, Journal Citation Reports also provides measures of more immediate and more prolonged journal impact.

Immediacy index: average impact of articles during the year in which they are published. 

5-year impact factor: average impact of articles over a ~5-year time span.

The SCImago journal rank (SJR) indicator is based on the PageRank algorithm used by Google Search. It weights citations from higher ranked journals more heavily than citations from lower ranked journals, while also taking into account the closeness of journals in terms of their research topics. Citation data come from Scopus. 

SJR indicator: overall influence of a journal on the scientific communitiy over a ~3-year time span; normalized so that the SJR of the average journal in Scopus is 1. 

Eigenfactor metrics are based on the PageRank algorithm used by Google Search. Rather than assuming that all citations are of equal value, citations from higher ranked journals are weighted more heavily than citations from lower ranked journals. Citation data come from Journal Citation Reports. 

Eigenfactor score: overall importance of a journal to the scientific community over a ~5-year time span; scaled so the sum of eigenfactor scores of all journals in the Journal Citation Reports database is 100.

Article Influence score: average influence of a journal's articles over a ~5-year time span; normalized so that the article influence score of the mean article in the Journal Citation Reports database is 1.