Fractional galaxy overdensity around BCG candidates selected with the reference “BCG-500” algorithm (top panels). The bottom panels show the difference in galaxy overdensity between various BCG selection algorithms and the reference. The left-hand column shows the results for all clusters, the middle one for poor clusters only and the right-hand panel for rich clusters only. The error on the difference is simply approximated by the error on the reference overdensity measurement – the true error will only be slightly larger due to the large covariance of the results. The “BCG-500+” leads to the highest, most concentrated peak, indicating that these BCGs are closest to the centre of the cluster member distribution.