Table 3. Model comparison based on the Bayesian Information Criterion and random effects EM estimation.

Key: ‘b’ return sensitivity, so that ‘bb’ refers to a model with separate appetitive and aversive sensitivity; ‘a’ learning rate; ‘p’ Pavlovian bias; ‘l’ lapse rate; ‘g’ Go-bias (favoring action over inaction). Separate iBIC for the two groups, and also the total sample, are shown in columns. Using a single set of parameters to describe the group distributions for the healthy and control groups achieves a better score, by about 61 BIC units, than summing the best of each separate fit. Were one to fit the two groups separately, one would run a danger of over-fitting, here over-emphasizing differences between the groups.

	Healthy control (separately)	Depression group (separately)	Common group-level parameters
Core model (bbaplg)	3623.96	5866.12	9411.59
Core model minus go-bias (bbapl)	3690.58	6188.85	9808.36
Core model minus Pavlovian bias (bbalg)	3606.30	5912.36	9443.71
bbal	3674.52	6224.73	9849.59
bapl	3792.21	6399.70	10143.98
baplg	3688.10	5977.05	9594.96
balg	3676.90	6092.47	9706.65
bal	3773.06	6499.39	10232.58