  We study how well we can retrodict results of measurements made on a quantum
system if we can make measurements on its final state. We know what
measurements were made, but not their results. An initial examination shows
that we can gain anywhere from no information to perfect information about the
results of previous measurements, depending on the measurements and the initial
state of the system. The case of two two-outcome measurements, the second of
which is a projective measurement, is examined in some detail. We then look at
a model of a qubit interferometer in which measurements are made in order to
determine the path the qubit followed. The measurement made on the final state
of the qubit depends on the information about previous measurement results that
we are trying to determine. One can attempt to find the result of just one of
the measurements, all of them, or find a measurement sequence that was not
realized. We study all three possibilities.
