  Collaborative work and co-authorship are fundamental to the advancement of
modern science. However, it is not clear how collaboration should be measured
in achievement-based metrics. Co-author weighted credit introduces distortions
into the bibliometric description of a discipline. It puts great weight on
collaboration - not based on the results of collaboration - but purely because
of the existence of collaborations. In terms of publication and citation
impact, it artificially favors some subdisciplines. In order to understand how
credit is given in a co-author weighted system (like the NRC's method), we
introduced credit spaces. We include a study of the discipline of physics to
illustrate the method. Indicators are introduced to measure the proportion of a
credit space awarded to a subfield or a set of authors.
