Upper panel: cumulative cluster mass function for the cosmological model best fitting the REFLEX Survey data and its uncertainties (grey region). The present data are compared to a previous determination of the cumulative mass function of clusters by Bahcall & Cen (1993), shown by data points. The different data points relate to different cluster samples used by Bahcall & Cen: Abell clusters (triangles with error bars), EDCC clusters (squares), and X-ray clusters (circles with error bars). The dotted curve shows the best fitting Schechter function, which falls short of describing the high end of the mass function. Bottom panel: ratio of the fitted function to the true cumulative mass function. We also show the uncertainties of the mass function from the marginalisation not including the varying Hubble parameter (dashed lines). The dotted lines indicate 10% deviations.