Left: run of the strength of the molecular indices of MS stars in NGC 2158 vs. their V magnitudes. A line has been fitted (dashed line) to remove the temperature and gravity dependence and to obtain the corrected pseudo-indices. Right: normal (histogram) and generalized (solid line) distributions of each corrected pseudo-index. Dashed lines are the best single Gaussians fitted to each generalized distribution. Note that the dashed lines fall so closely to the solid lines in all the cases that it is difficult to distinguish them. The residuals between the single Gaussian and the generalized distribution are shown in the inset panels.