Differences in astrometric uncertainties between those given in the Gaia DR2 catalogue and those estimated by the ASF of Gaia DR2. Panel a) shows the differences in the uncertainties of RA and Dec coordinates (epoch J2015.5). Panel b) shows the differences in the uncertainties of proper motion along RA and Dec coordinates. Panel c) shows the differences in the uncertainties of parallax. Panels a) and b) separately include the values along RA and Dec coordinates, and thus the number of data points is twice that in panel c). The data consist of the S1, S3, and S4 samples of our ‘correlation’ method (Appendix A).