  We study the possibility to control systematic errors of the ground-state
parameters obtained by Shifman-Vainshtein-Zakharov (SVZ) sum rules, making use
of the harmonic-oscillator potential model as an example. In this case, one
knows the exact solution for the polarization operator, which allows one to
obtain both the OPE to any order and the parameters (masses and decay
constants) of the bound states. We determine the parameters of the ground state
making use of the standard procedures of the method of QCD sum rules, and
compare the obtained results with the known exact values. We show that in the
situation when the continuum contribution to the polarization operator is not
known and is modelled by an effective continuum, the method of sum rules does
not allow to control the systematic errors of the extracted ground-state
parameters.
