*Internal consistency:*
The astrometric processing is essentially a weighted least-squares solution and statistical
tests of the residuals (including graphical output such as histograms, sky maps,
and scatter plots) can be powerful method to monitor the quality and progress of the data
processing and detect potential problems. Ideally, the (weighted) residuals should be
unbiased, uncorrelated, Gaussian, independent of other variables, and consistent with
calculated uncertainties.