  We show that including both the system and the apparatus in the quantum
description of the measurement process, and using the concept of conditional
probabilities, it is possible to deduce the statistical operator of the system
after a measurement with a given result, which gives the probability
distribution for all possible consecutive measurements on the system. This
statistical operator, representing the state of the system after the first
measurement, is in general not the same that would be obtained using the
postulate of collapse.
