Correlation between uniform mock reference and comparison tracers. The color indicates the true classification following the color-coding in Fig. 1: blue for a, orange for b. The right panel of each row shows the marginalized distribution of each quadrant, following the color-coding of the circles. The light histograms correspond to the off-diagonal terms: false a in light orange, false b in light blue. Top panels: perfect and noise-free tracer, i.e., without any off-diagonal points; middle panels: imperfect (c_a = 15% and c_b = 25%) yet noise-free tracer, the sum of its off-diagonal terms corresponds to its contamination parameters; bottom panels: perfect but noisy tracer. In this case, plain off-diagonal fractions are 11% in false a and 7.5% in false b, while both should be 0% for a perfect tracer; the likelihood estimator (Eq. (11), see Sect. 2.4), properly accounting for the fraction of off-diagonal terms caused by measurement errors, provides contamination estimates and , compatible with zero.