  Closed expressions are derived for the quantum measurement statistics of
pre-and postselected gaussian particle beams. The weakness of the pre-selection
step is shown to compete with the non-orthogonality of post-selection in a
transparent way. The approach is shown to be useful in analyzing
post-selection-based signal amplification, allowing measurements to be extended
far beyond the range of validity of the well-known Aharonov-Albert-Vaidman
limit.
