b-parameter versus age of the young population in the starburst candidate sample divided into three groups in luminosity indicated at the upper right. Each group contains about 2000 galaxies. The starburst criterion b = 3 is shown with a red horizontal line. Also displayed in three tracks are the minimum b parameter values needed to reach f_burst = 3% at the age in question. In this case three alternatives of the SFH are displayed: a constant SFR (solid blue line) and exponentially decaying SFR:s on timescales of 10 and 100 Myr (red dashed line and green dash-dotted line respectively). The hexagons show the surface density of data points. Darker − more data.