Stellar population synthesis can be approached as an inverse problem.
The physical information is extracted from the observations through an
inverse model.
The process requires the transformation of the observational errors into
model errors.
A description is given for the error analysis to obtain objectively the
errors in the model. Finding a solution for overdetermined and under-determined
case was the purpose of two preceding papers. This new one completes the
problem of stellar populations synthesis by means of a data base, by providing
practical formul\ae defining the set of acceptable solutions. All solutions
within this set are compatible, at a given confidence level, with the
observations.Comment: 11 pages, LaTeX, 4 figures, 1 table. M.N.R.A.S.(2000) in pres