Test of the reliability of the SEM mass fraction determination. Synthetic spectra were fed into our SEM model and the derived mass fraction of the burst (ordinate) versus the input mass fraction (abscissa) are displayed. The same conditions for modelling holds as in Fig. A.4. At each time step there are 8 models with different ages of the starburst: 5.5, 10, 25, 55, 95, 250, 550 and 950 Myr. Larger symbol: higher age.