An article in Nature today on attempts to standardize impact factors across academic disciplnes today – a brief quote from the article states:
Performance metrics based on values such as citation rates are heavily biased by field, so most measurement experts shy away from interdisciplinary comparisons. The average biochemist, for example, will always score more highly than the average mathematician, because biochemistry attracts more citations. But researchers at Indiana University Bloomington think that they have worked out the best way of correcting this disciplinary bias. And they are publishing their scores online, for the first time letting academics compare rankings across all fields.