Impact Factor based on 5 years of history. Copyright Thomson Reuters.
A base of five years may be more appropriate for journals in certain fields because the body of citations may not be large enough to make reasonable comparisons, or it may take longer than two years to publish and distribute leading to a longer period before others cite the work.
Actual value is intentionally only displayed for the most recent year. Earlier values are available in the Journal Citation Reports from Thomson Reuters.
Journals are often ranked by Impact Factor in an appropriate Thomson Reuters subject category. As there are now two Impact Factors published – two-year and five-year Impact Factors – this rank may differ, so care is needed when assessing these ranked lists to understand which of the two metrics is being used. In addition, journals can be categorized in multiple subject categories, giving them different ranks for each subject. Consequently, a rank should always be in context to the subject category.