Measuring national output in physics: Delimitation problems

In a study of the Dutch publication output in physics we tested methods of delimitating fields by journal categories in the Science Citation Index (SCI) compared to the classification of individual publications into subfields in the subject specific database Physics Briefs (PHYS). Different methods of measuring national scientific output were compared as well. In this paper we report the main findings on these issues, based on a study of six selected subfields in physics. The main conclusion with respect to the use of different classification methods is that in most of the selected fields in physics the method which delimitates fields by journal categories yields an incomplete picture of the output of a country. Particularly because this method neglects a considerable number of articles published in general journals. With respect to different methods of counting publications it was corroborated by the Dutch data in Physics Briefs that: 1. so-called 'integer counted' world shares are very much influenced by the degree of 'internationalisation' and 2. 'first author counting' gives a satisfactory approximation of 'fractional counting'. Citation indicators based on 'first author counting', however, may be distorted in fields with a large fraction of international co-authored publications.