X-ray luminosity-richness scaling. The solid line indicates the mean fitted regression line of log L_X, measured within a 500 kpc aperture, on log n200, while the dashed line shows this mean plus or minus the intrinsic scatter σ_scat. The shaded region marks the 68% highest posterior credible interval for the regression. Error bars on the data points represent observed errors for both variables (computed following the usual astronomical practice). The distances between the data and the regression line is partly caused by the measurement error and partly by the intrinsic scatter. The upper abscissa indicates the cluster mass.