Number of PMS members (upper panel) and ratio of PMS to total member numbers (PMS+MS+postMS), plotted versus cluster age. The values for NGC 2264 (crossed circle) and NGC 1893 (star) are shown for comparison. The green line in the lower plot represents a linear fit to the points, excluding the clusters marked in red with their names in the upper plot. A correlation coefficient of 0.7 is obtained. Error bars in the age are listed in Table 1. In the vertical axis, they are calculated assuming absolute error in the numbers equal to the square root of the numbers themselves.