Radial velocity uncertainties tested with open clusters. Top panel: absolute value of the difference between the radial velocity of a star and its cluster median |ΔRV| normalised by the radial_velocity_ error. The black line is the lowess (locally weighted scatterplot smoothing). The slope of the lowess for lower values of radial_velocity_error indicates that the errors can be underestimated at the bright end (but see the text for a discussion). Bottom panel: difference between the radial velocity of a star and its cluster median ΔRV normalised by the radial velocity error in different radial velocity error bins.