2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/90357The 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.3 pagesQuantum Physicstext