  We discuss the extraction of ground-state parameters, such as decay constants
and form factors, from two- and three-point dispersive sum rules, making use of
a quantum-mechanical potential model. This model provides a unique possibility
to probe the reliability and the accuracy of the method of sum rules: one
obtains the bound-state parameters by the standard procedures adopted in sum
rules, and compares these results with the exact values, known in the potential
model. We demonstrate the presence of uncontrollable uncertainties in the
ground-state parameters obtained from sum rules and estimate their magnitude.
The uncontrolled uncertainties in ground-state form factors are shown to be
typically much larger than those in the decay constants. In the example
presented, the uncontrolled systematic error in the extracted form factor is
found to exceed the 10% level.
