  One of the critical issues in bibliometric research assessments is the time
required to achieve maturity in citations. Citation counts can be considered a
reliable proxy of the real impact of a work only if they are observed after
sufficient time has passed from publication date. In the present work the
authors investigate the effect of varying the time of citation observation on
accuracy of productivity rankings for research institutions. Research
productivity measures are calculated for all Italian universities active in the
hard sciences in the 2001-2003 period, by individual field and discipline, with
the time of the citation observation varying from 2004 to 2008. The objective
is to support policy-makers in choosing a citation window that optimizes the
tradeoff between accuracy of rankings and timeliness of the exercise.
