  [Abridged] The WIde-field Nearby Galaxy-cluster Survey (WINGS) is a project
aiming at the study of the galaxy populations in clusters in the local universe
(0.04<z<0.07) and the influence of environment on the physical properties of
galaxies. This survey provides a high quality set of spectroscopic data for
~6000 galaxies in 48 clusters. A salient feature of this model is the
possibility of treating dust extinction as a function of age, allowing younger
stars to be more obscured than older ones. Our technique, for the first time,
takes into account this feature in a spectral fitting code. A set of template
spectra spanning a wide range of star formation histories is built, with
features closely resembling those of typical spectra in our sample in terms of
spectral resolution, noise and wavelength coverage. Our method of analyzing
these spectra allows us to test the reliability and the uncertainties related
to each physical parameter we are inferring. The well-known degeneracy problem,
i.e. the non-uniqueness of the best fit solution (mass and extinction in
different age bins), can be addressed by assigning adequate error bars to the
recovered parameters. The values found in this way, together with their error
bars, identify the region of parameter space which contains all the possible
solutions for a given spectrum. A comparison test was also performed on a WINGS
subsample, containing objects in common with the Sloan Digital Sky Survey,
yielding excellent agreement. We find that the stellar content as a function of
age is reliably recovered in four main age bins and that the uncertainties only
mildly depend on the S/N ratio. The metallicity of the dominant stellar
population is not always recoverable unambiguosly, depending on the Star
Formation History pattern.
