  National research assessment exercises are conducted in different nations
over varying periods. The choice of the publication period to be observed has
to address often contrasting needs: it has to ensure the reliability of the
results issuing from the evaluation, but also reach the achievement of frequent
assessments. In this work we attempt to identify which is the most appropriate
or optimal publication period to be observed. For this, we analyze the
variation of individual researchers' productivity rankings with the length of
the publication period within the period 2003-2008, by the over 30,000 Italian
university scientists in the hard sciences. First we analyze the variation in
rankings referring to pairs of contiguous and overlapping publication periods,
and show that the variations reduce markedly with periods above three years.
Then we will show the strong randomness of performance rankings over
publication periods under three years. We conclude that the choice of a three
year publication period would seem reliable, particularly for physics,
chemistry, biology and medicine.
