Joint likelihood contours (95.6% and 68.7%) for couples of best-fit parameters for the experimental configuration EC0 (ground based observations with a 4-band photometer) where the data are analysed by fitting the three parameters τ_t, T_e, ΔI_d. The filled contours are for a Gaussian prior on T_e with σ = 3   keV; the dashed contours are for a Gaussian prior with σ = 8   keV. The * symbols mark the input values of parameters. In this case a significant bias for all parameters is evident. The anti-correlation between τ_t and T_e is also evident.