Bibliometric indicators are increasingly used in support of decisions about recruitment, career advancement, rewards, and selective funding for scientists. Given the importance of the applications, bibliometricians are obligated to carry out empirical testing of the robustness of the indicators, in simulations of real contexts. In this work, we compare the results of national-scale research assessments at the individual level, based on the following three different indexes: the h-index, the g-index, and “fractional scientific strength” (FSS), an indicator previously proposed by the authors. For each index, we construct and compare rankings lists of all Italian academic researchers working in the hard sciences during the period 2001–2005. The analysis quantifies the shifts in ranks that occur when researchers' productivity rankings by simple indicators such as the h- or g-indexes are compared with those by more accurate FSS.