Variation of parameter B of the scaling function for uncertainty in RV (S_RV) with target age for observations with order-sorting filter HR15N. Square symbols show the value of B for targets identified as possible cluster members from their RV versus the nominal age of the cluster (see Table 1). No selection by RV is made for NGC 6633 or COROT and we assume the stars have mean ages > 1 Gyr. The dotted line indicates the median value of T_eff (right hand axis values) for members identified in each cluster.