Upper diagrams: our model predictions of EW_Hα,em as function of the duration of the burst when 3 and 30% of the total mass is formed during a burst with constant SFR. Two different scenarios are displayed: 1) a burst in an old galaxy where the stars were formed in a single burst (hatched line); and 2) a burst in a galaxy which has had a constant SFR over a Hubble time (solid line). Lower diagrams: the evolution of the b-parameter. The limit of our criterion of a starburst, the soft criterion, stating that , is marked with the horizontal line in the lower diagrams. The blue inclined line corresponds to mass fractions of 3 and 30% respectively. The vertical lines indicate the maximum ages of a starburst with mass fractions 3 and 30% respectively, to fulfil the soft starburst criterion. The horizontal lines in the upper diagrams indicate EW_Hα,em at that burst duration and the corresponding EW_Hα,em in numbers are given below. The shaded area indicates a region containing starbursts according to the b-parameter criterion but which will not be powerful enough to produce postbursts according to the EW_Hδ,abs< −6 Å criterion.