  The growing focus in research policy worldwide on top scientists makes it
increasingly important to define adequate supporting measures to help identify
excellent scientists. Highly cited publications have since long been associated
to research excellence. At the same time, the analysis of the high-end of
citation distributions still is a challenging topic in evaluative
bibliometrics. Evaluations typically require indicators that generate
sufficiently stable results when applied to recent publication records of
limited size. Highly cited publications have been identified using two
techniques in particular: pre-set percentiles, and the parameter free
Characteristic Scores and Scales (CSS) (Gl\"anzel & Schubert, 1988). The
stability required in assessments of relatively small publication records,
concerns size as well as content of groups of highly cited publications.
Influencing factors include domain delineation and citation window length.
Stability in size is evident for the pre-set percentiles, and has been
demonstrated for the CSS-methodology beyond an initial citation period of about
three years (Gl\"anzel, 2007). Stability in content is less straightforward,
considering for instance that more highly cited publications can have a later
citation peak, as observed by Abt (1981) for astronomical papers. This paper
investigates the stability in content of groups of highly cited publications,
i.e. the extent to which individual publications enter and leave the group as
the citation window is enlarged.
