  We derive an expression for the nearest-neighbor spacing distribution $P(s)$
of the energy levels of quantum systems with intermediate dynamics between
regularity and chaos and missing levels due to random experimental errors. The
expression is based on the Brody distribution, the most widely used for fitting
mixed spectra as a function of one parameter. By using Monte Carlo simulations
of intermediate spectra based on the $\beta$-Hermite ensemble of Random Matrix
Theory, we evaluate the quality of the formula and its suitability for fitting
purposes. Estimations of the Brody parameter and the fraction of missing levels
can be obtained by a least-square two-parameter fitting of the experimental
$P(s)$. The results should be important to distinguish the origins of
deviations from RMT in experimental spectra.
