Logarithm of the average trigonometric parallax of clusters log ϖ_TGAS obtained from individual parallaxes of cluster members vs. logarithm of their photometric parallaxes log ϖ_MWSC. The sizes of the circles are proportional to the number of TGAS parallaxes available for the calculation of the mean parallax; the vertical bars represent the mean error of the mean. The open circles indicate four discordant clusters discussed in the text. The pluses represent the TGAS parallaxes for selected nearby clusters from vL2017, which are shown here for comparison. The red line indicates a linear regression computed. The dotted line indicates the bisector.