Ambiguities in the derivation of retrodictive probability
  The derivation of the quantum retrodictive probability formula involves an error, an ambiguity. The end result is correct because this error appears twice, in such a way as to cancel itself. In addition, however, the usual expression for the probability itself contains the same ambiguity; this may lead to errors in its application. A generally applicable method is given to avoid such ambiguities altogether. 
K. A. Kirkpatrick