  Individual, excellent scientists have become increasingly important in the
research funding landscape. Accurate bibliometric measures of an individual's
performance could help identify excellent scientists, but still present a
challenge. One crucial aspect in this respect is an adequate delineation of the
sets of publications that determine the reference values to which a scientist's
publication record and its citation impact should be compared. The structure of
partition cells formed by intersecting fixed subject categories in a database
has been proposed to approximate a scientist's specialty more closely than can
be done with the broader subject categories. This paper investigates this cell
structure's suitability as an underlying basis for methodologies to assess
individual scientists, from two perspectives: (1) Proximity to the actual
structure of publication records of individual scientists: The distribution and
concentration of publications over the highly fragmented structure of partition
cells are examined for a sample of ERC grantees; (2) Proximity to customary
levels of accuracy: Differences in commonly used reference values (mean
expected number of citations per publication, and threshold number of citations
for highly cited publications) between adjacent partition cells are compared to
differences in two other dimensions: successive publication years and
successive citation window lengths. Findings from both perspectives are in
support of partition cells rather than the larger subject categories as a
journal based structure on which to construct and apply methodologies for the
assessment of highly specialized publication records such as those of
individual scientists.

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