The effect of applying different preselection criteria to improve the detection fraction of calibrator sources, including a lower limit on flux density (left panel), lower limit on flux density plus requiring a low-frequency spectral turnover (middle panel), or lower limit on flux density plus a lower limit to spectral index as calculated between VLSSr and WENSS (right panel). The three colours correspond to imposing the lower flux density limit on the value obtained from VLSSr (orange), WENSS (black), or NVSS (blue). In each panel, the dashed line shows the fraction of the total sample that remains as the lower limit to flux density is raised, while the solid line shows the fraction of that remaining sample that are good calibrators. Imposing a spectral turnover or low-frequency spectral index requirement can reduce the sample size by a factor of 10 whilst still discovering almost half of the total acceptable calibrators.