Chi-square test for nested models which were estimated from Weibull functions.

M1 is the full model with both slope and threshold varying across the 40 PFs; M2 is the reduced model that has the same slope across the 40 PFs; M3 is another reduced model with the same threshold across the 40 PFs; M4 is the most reduced model that has the same slope and threshold in all 40 PFs; df is the degree of freedom. For all four observers, M2 is the only model that is comparable to M1 (the full model) and superior to M4 (the most reduced model).