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.