  We present a path analysis of the condition under which the outcomes of
previous observation affect the results of the measurements yet to be made. It
is shown that this effect, also known as "signalling in time", occurs whenever
the earlier measurements are set to destroy interference between two or more
virtual paths. We also demonstrate that Feynman's negative "probabilities"
provide for a more reliable witness of "signalling in time", than the
Leggett-Garg inequalities, while both methods are frequently subject to failure
