The h-index seems to be breaking away from the bibliometric pack, in the race to become a favoured measure of scientific performance ('Achievement index climbs the ranks' Nature 448, 737; 2007). However, if the h-index is to become an assessment tool commonly used by university administrators and government bureaucrats, those using it should be aware of its pitfalls.

As noted in your News story, tallying how many papers a researcher publishes (their productivity) gives undue merit to those who publish many inconsequential papers. But at least for ecologists and evolutionary biologists, the h-index is highly correlated with productivity (r = 0.77; see C. D. Kelly and M. D. Jennions Trends Ecol. Evol. 21, 167–170; 2006).

This is worrisome, because the h-index is easily misconstrued as an equitable measure of research quality. We offer two examples.

First, female ecologists and evolutionary biologists publish fewer papers than their male counterparts, and they have significantly lower h-indices. Should administrators therefore conclude that men are better researchers? No. The gender difference vanishes if we control for productivity. It seems unlikely that this phenomenon is restricted to ecology and evolution.

Second, the h-index increases with age and using the ratio of the two can be problematic. Therefore, reliably comparing the performance of younger researchers with older ones is difficult.