To quote Thomsons "a journal's Impact factor is calculated by dividing the number of current year citations to the source items published in that journal during the previous two years."
I assume that disciplines vary in
- average number of citations per paper: Disciplines with fewer citations per paper will appear to have less impact.
- citation half-life: Longer half-lives means under-estimation of impact relative to journals with shorter half-lives. The Wikipedia article on impact factors summarises a study that found that "the percentage of total citations occurring in the first two years after publication varies highly among disciplines from 1-3 percent in the mathematical and physical sciences to 5-8 percent in the biological sciences." (Nierop, 2009).
Google Scholar uses the five year h-index. See this listing of top ranked journals with various psychology related keywords in their title. The five year h-index indicates the number of papers with an equivalent number of citations. E.g., a value of 20 indicates that 20 articles published in the last five years have received 20 or more citations.
However, while the h-index might reduce the issue of different citation half-lives, it does not resolve it. And it does not address the issue of differential citation patterns across disciplines.
What index provides both a reliable and unbiased assessment of the citation based impact of a journal when comparisons are being performed across disciplines?
- Erjen van Nierop (2009). "Why do statistics journals have low impact factors?". Statistica Neerlandica 63 (1): 52–62. doi:10.1111/j.1467-9574.2008.00408.x.