  The quantum-mechanical constraints on the information transfer in the
measuring devices and systems and their influence on measurement outcomes are
studied. As the model example, the measurement of dichotomic observable A,
describing the object S, is performed by the information system O during S,O
interaction. The measurement of two S ensembles is considered, first one is the
pure ensemble, in which S state is the superposition of A eigenstates |S1>,|S2>
other one is their probabilistic mixture. It is shown that these information
constraints induced by Heisenberg commutation relations bloke the transfer of
information about the purity of S state; for the studied ensembles it is
described by the expectation value of observable A', conjugated to A. Due to
these restrictions, O can't discriminate the pure and mixed S ensembles with
the same A expectation value. We demonstrate that this information losses
result in the appearance of stochasticity in the measurement of such S pure
ensemble, so that in the individual events O would detect the random 'pointer'
values O1,O2, which describe the outcome for O measurement of incoming A
eigenstates |S1,2>. The influence of O decoherence by its environment is
studied, it's shown that the account of its effects doesn't change this results
principally.
