Comparing Gaia and predicted parallaxes to test either the distance scale or the parallax zero point. Shown is the change in χ² caused by a multiplicative term in the photometric parallaxes (left panel) or an additive term in DR1 parallaxes (right panel). In each panel, the dashed curve uses the nominal errors, while the solid curve is for errors scaled by a factor 0.80, as suggested in order to obtain χ² = 1 per degree of freedom. See text for more discussion.