Comparison between the simulations results for 20% (dashed), 30% (solid), and 40% (dotted) SFE for model LK 1 and the observed properties of young massive loose clusters (circles) as summarised in Pfalzner (2009). a) shows the relation between cluster age and size r_c; b) the cluster mass; and c) the cluster density as function of cluster size, respectively. The colours indicate cluster age (red t_c < 4 Myr, green 4 Myr < t_c < 10 Myr, blue t_c > 10 Myr), where in the simulations it was assumed that the cluster had an age of 1 Myr at the time of instantaneous gas expulsion.