  Assessing the research performance of multi-disciplinary institutions, where
scientists belong to many fields, requires that the evaluators plan how to
aggregate the performance measures of the various fields. Two methods of
aggregation are possible. These are based on: a) the performance of the
individual scientists or b) the performance of the scientific fields present in
the institution. The appropriate choice depends on the evaluation context and
the objectives for the particular measure. The two methods bring about
differences in both the performance scores and rankings. We quantify these
differences through observation of the 2008-2012 scientific production of the
entire research staff employed in the hard sciences in Italian universities
(over 35,000 professors). Evaluators preparing an exercise must comprehend the
differences illustrated, in order to correctly select the methodologies that
will achieve the evaluation objectives.
