Bias in the parallax difference as a function of the apparent magnitude. Two mock stars are considered for the asteroseismic bias: a RGB star (blue) and a RC star (red; see text for details). We show the asteroseismic bias that would result from a 1–3% systematic uncertainty in radius. The Gaia bias, ±10–40 μas in parallax, is shown as a yellow hatched region. Kepler observations are shown in the background after subtracting the mean parallax offset.