Test of the reliability of the SEM age determination. Synthetic spectra were fed into our SEM model and the derived age of the burst (ordinate) versus the input age (abscissa) are displayed. The input model has a decay rate of 1 Gyr but there are two different choices for the star formation history and metallicity in the fit, either with a decay rate of 100 Myr or one with 1 Gyr and a metallicity of Z = 0.004 or Z = 0.008. At each time step there are 10 models with different mass fractions: 2, 4, 6, 8, 10, 20, 40, 60, 80 and 100%. Larger symbol: larger mass fraction.