  A first systematic study of the effects of the choice of the input scale in
global determinations of parton distributions and QCD parameters is presented.
It is shown that, although in principle the results should not depend on these
choices, in practice a relevant dependence develops as a consequence of what is
called procedural bias. This uncertainty should be considered in addition to
other theoretical and experimental errors, and a practical procedure for its
estimation is proposed. Possible sources of mistakes in the determination of
QCD parameter from parton distribution analysis are pointed out.
