Relation between the rms deviation of the men PMS age (Σ) and the size of the clusters, given by the characteristic clustering scale (Δ. See text) for PMS members of masses above the approximate completeness limit (see Sect. 3.1.2). Horizontal error bars are calculated with the assumption of a typical error in the distance modulus of 0.2. A bootstrapping estimate of the uncertainty in Σ gives values similar to the size of the representative points. The straight line reproduces the slope (0.33) of the scaling relation between age spread and size proposed by Efremov & Elmegreen (1998) for star-forming regions in a wide range of sizes. The green line shows a linear least squares fit, excluding the clusters marked with their names. A slope of 0.31 and a correlation coefficient of 0.8 are obtained.