Defining subject collections for informetric analyses: The effect of varying the subject aboutness level

Subject literature collections are typically formed by judgements which are inexplicit and imprecise. This seems to compromise the worth of precise measurements made of their properties. In this paper an examination is made of how several commonly-measured properties of subject literatures vary as an important factor in the compilation of subject collections is varied. The factor is the amount which a document must 'say' about a subject for it to be included in such a collection. This document property has been expressed in formal terms and given a simple measure for the one subject examined, the research topic of Bradford's Law of Scattering. It is found that lowering the level of subject aboutness required for admission to a collection produces a large increase in the size of the collection obtained, and an appreciable change in some size-related properties. For these properties, the initial concern is warranted. However, other parameters are found to be invariant to such changes.