  When planning new facilities, one is interested to know whether and by how
much the new technique is superior to already existing ones. We describe a
general approach permitting to evaluate the relative merits of various
techniques used in astrophysical observations, following the theory of model
parameter estimation. It is applied to compare two-aperture optical long
baseline interferometry to classical spectroscopy, both used to define the
model parameters of the P Cyg wind. The wind modeling was done using an
efficient approximation for computation of the line source function; it allowed
to analyse about 10e5 points in the parametric space of P Cyg envelope models.
It is shown that interferometry offers no real advantage if the object can be
described by stationary spherically symmetric models with a priori fixed
thermal structure. However, if the object must be described by a model with a
large number of free parameters, e.g. when the thermal structure of the
envelope is not fixed a priori, then the interferometric measurements can
reduce the error in model parameters determination by an order of magnitude. In
the case of P Cyg, the reduction of errors provided by interferometry is
highest for the baseline lengths in the range 45--90 m. This illustrates the
capacity of the proposed method to be used for optimization of interferometric
configurations. The simplifications adopted for this first trial application
are critically analyzed, and future improvements are indicated.
