# 14.7.3 Radial velocity validation

We use clusters to validate the uncertainty on radial velocity, radial_velocity_error. Using 48 944 stars in 804 clusters that have at least 10 members with radial velocities. I have computed the difference between the RV of each star and the bulk cluster RV (defined as the median). In the ideal case, the stars should be on average 1 error bar from the expected value.

But the bright stars (with high rv_expected_sig_to_noise and small radial_velocity_error) tend to be on average much more than 1 error bar from the median. This is not due to a resolved velocity dispersion, the effect seems to be the same in all clusters, and it would indicate that all clusters have internal velocity dispersions larger than 1 $\mathrm{km}{\mathrm{s}}^{-1}$. In practice, the stars with very small nominal uncertainties are 5 to 10 error bars away from the cluster velocity.

The nominal uncertainties for stars with errors from 2 $\mathrm{km}{\mathrm{s}}^{-1}$ to 10 $\mathrm{km}{\mathrm{s}}^{-1}$ seem realistic (see Figure 14.91).