What is a fair metric for assessing the citation impact of journals across disciplines? 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.

Question

What index provides both a reliable and unbiased assessment of the citation based impact of a journal when comparisons are being performed across disciplines?

Reference


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.
There is no such index.

Publication and citation standards vary significantly between disciplines and even sub-disciplines.  Without direct, deep knowledge of the standards in each community, it is simply impossible to compare impact of a journal in field X with the impact of a journal in field Y.  (Eigenfactor's extraordinary claims to the contrary require extraordinary evidence, which they don't provide.)

Moreover, it's not clear why you should even try.  Any judgement about the relative importance of Journal of X versus Journal of Y necessarily requires a prior judgement about the relative importance of field X versus field Y.  HC SVNT DRACONES.