  Accurate measurement of research productivity should take account of both the
number of co-authors of every scientific work and of the different
contributions of the individuals. For researchers in the life sciences, common
practice is to indicate such contributions through position in the authors
list. In this work, we measure the distortion introduced to bibliometric
ranking lists for scientific productivity when the number of co-authors or
their position in the list is ignored. The field of observation consists of all
Italian university professors working in the life sciences, with scientific
production examined over the period 2004-2008. The outcomes of the study lead
to a recommendation against using indicators or evaluation methods that ignore
the different authors' contributions to the research results.
