Minimum purity of the published alert stream as a function of Galactic latitude (see text for details). The behaviour as a function of Galactic latitude may be understood by considering the coverage of other transient surveys which generally avoid the plane, are not full sky and are biased towards northern skies (ASAS-SN is the only other all sky transient survey). Additionally, very few alerts towards the Galactic plane are followed up. The hatched regions show ranges in Galactic latitudes which are used in the purity analysis (Sects. 4.1.1 and 4.1.3). The horizontal hatched region contains 1661 published alerts and has an overall minimum purity of 0.69. The diagonal hatched region covering ± 8 degrees around the Galactic plane contains 1628 published alerts and has an overall minimum purity of 0.09.