  Multiparametric statistical model providing stable reconstruction of
parameters by observations is considered. The only general method of this kind
is the root model based on the representation of the probability density as a
squared absolute value of a certain function, which is referred to as a psi
function in analogy with quantum mechanics. The psi function is represented by
an expansion in terms of an orthonormal set of functions. It is shown that the
introduction of the psi function allows one to represent the Fisher information
matrix as well as statistical properties of the estimator of the state vector
(state estimator) in simple analytical forms.
