Tutt Library Research Guides
Are Citations the Best Measure of History Journals?
...that number is calculated by taking the number of citations to articles in the journal over the previous two years and dividing it by the number of citations to those articles in the (curent year).
The report acknowledges this, noting that more than half of the citations to history journal articles were published ten years ago or longer.
See more at: News, Community, and Historical Thinking, blog of the American History Association.