Comparison between the estimated of the WH15 cluster member galaxies within each bin of transverse distance and the theoretical predictions from GR (solid blue line), f(R) (dotted red line), and sDGP (dash-dotted green line). The shaded coloured areas show the model errors that were caused by the fitting uncertainties on the cluster mass distribution, and the dispersion of the cluster redshifts. The black points show the estimated . The vertical error bars represent the range of the parameter, containing 68% of the marginalised posterior probability, while the horizontal error bars show the dispersion of the galaxy transverse distances in a given bin.