  PP(top x%) is the proportion of papers of a unit (e.g. an institution or a
group of researchers), which belongs to the x% most frequently cited papers in
the corresponding fields and publication years. It has been proposed that x% of
papers can be expected which belongs to the x% most frequently cited papers. In
this Letter to the Editor we will present the results of an empirical test
whether we can really have this expectation and how strong the deviations from
the expected values are when many random samples are drawn from the database.
