  Because of the variations in citation behavior across research fields,
appropriate standardization must be applied as part of any bibliometric
analysis of the productivity of individual scientists and research
organizations. Such standardization involves scaling by some factor that
characterizes the distribution of the citations of articles from the same year
and subject category. In this work we conduct an analysis of the sensitivity of
researchers' productivity rankings to the scaling factor chosen to standardize
their citations. To do this we first prepare the productivity rankings for all
researchers (more than 30,000) operating in the hard sciences in Italy, over
the period 2004-2008. We then measure the shifts in rankings caused by adopting
scaling factors other than the particular factor that seems more effective for
comparing the impact of publications in different fields: the citation average
of the distribution of cited-only publications.
