  Scientific output varies between research fields and between disciplines
within a field such as astrophysics. Even in fields where publication is the
primary output, there is considerable variation in publication and hence in
citation rates. Data from the Smithsonian/NASA Astrophysics Data System is used
to illustrate this problem and argue against a "one size fits all" approach to
performance metrics, especially over the short time-span covered by the
Research Assessment Exercise (soon underway in the UK).
