Comparison of the parallaxes in TGAS and the Hipparcos Catalogue (nominal TGAS run, i.e., using the Hipparcos parallaxes as prior). Left: the normalized parallax errors (calculated minus the simulated true values, divided by their formal standard errors) are correlated. Right: the differences of the actual parallax values (normalized by their combined standard errors) follow a Gaussian distribution with standard deviation 0.91, less than 1.0 because of the correlation. The solid red line is a Gaussian distribution with unit width.