Radial scale length R_d of the thick disc versus metallicity. Considered are SEGUE thick disc stars with 7.5 kpc < R < 8.5 kpc, 4195 in total. Red points show the radial scale lengths calculated for each metallicity bin with the “effective” quantities in Eq. (9). The points are fitted with a constant and also with R_d linearly dependent on [Fe/H] (solid and dashed lines). One-sigma areas are shown in grey. The blue point corresponds to the value of scale length derived in case of binning the same sample in 4 |z|-bins.