Large increases and decreases in journal impact factors in only one year: The effect of journal self-citations

I studied the factors (citations, self-citations, and number of articles) that influenced large changes in only 1 year in the impact factors (IFs) of journals. A set of 360 instances of journals with large increases or decreases in their IFs from a given year to the following was selected from journals in the Journal Citation Reports from 1998 to 2007 (40 journals each year). The main factor influencing large changes was the change in the number of citations. About 54% of the increases and 42% of the decreases in the journal IFs were associated with changes in the journal self-citations.