  Within the impulse approximation the response of a many body system at large
momentum tranfer can be written in a simple and transparent form, allowing to
directly relate the inclusive scattering cross section to the properties of the
target ground state. Although the physics assumptions underlying impulse
approximation are well defined, their implementation involves ambiguities that
may cause significant differences in the calculated responses. We show that,
while minimal use of the impulse approximation assumptions naturally leads to
write the response in terms of the target spectral function, the widely used
alternative definition in terms of the momentum distribution involves a more
extended use of the same assumtpions. The difference between the responses
resulting from the two procedures is illustrated by two examples.
