The mass of the most massive star of each association is plotted as a function of the number of bona fide OB members (black symbols). Red diamonds represent the average M_up  of all associations with the same number of members, their error bars being the standard deviation. The general trend is that M_up  increases with the number of members. Note however that some poorly populated associations (3-5 members) also have very high upper masses, as in the case of Orion’s Trapezium. The most populated association, #151 with 69 members, deviates from the main trend.