Table2.

Predictors of interest in each model of VGT choice behaviour

Predictors of interest	Research question
Model 1. Group differences in the effect of EV ratio
EV ratio	Do controls use EV ratio information?
EV ratio ∗ group	Do the GD group differ in their sensitivity to EV ratio?
Intercept	At hypothetical EV ratio = 0, do controls show a preference for the high or low probability prospect?
Group	At hypothetical EV ratio = 0, do the GD group show a different preference to controls?
Model 2. Do potentially confounding clinical variables explain group differences in model 1?
EV ratio ∗ group	With clinical variables controlled for (clinical variable ∗ EV ratio ∗ group), do the effects of group on EV ratio survive?
Group	With clinical variables controlled for, does the group effect at EV = 0 survive?
Model 3. Do the groups differ in their use of probability and magnitude information?a
Probability ∗ group	Do the GD group use probability information more or less than controls?
Magnitude ∗ group	Do the GD group use magnitude information more or less than controls?
Model 4. Does previous trial feedback explain choice behaviour?
EV ratio ∗ Previous feedback	In controls, does the effect of EV ratio differ after a win (or zero outcome) compared to a zero outcome (or loss)?
EV ratio ∗ Previous feedback ∗ group	Does the effect of previous trial feedback differ in the GD group compared to controls?
EV ratio ∗ Previous feedback (group baseline reversed)b	In the GD group, does the effect of EV ratio differ after a win (zero outcome) compared to a zero outcome (loss).
Model 5. Within the GD group, do PGSI or GRCS scores predict the EV ratio effect?
PGSI ∗ EV ratio	Controlling for other clinical variables in Model 2, does gambling severity predict the EV ratio effect?
GRCS ∗ EV ratio	Controlling for other clinical variables in Model 2, do gambling cognitions predict the EV ratio effect?
Model 6. Does EV sensitivity vary as a function of task version (gain or loss)?c
EV ratio ∗ version	In controls, does the effect of EV ratio differ in the gain compared to the loss version?
EV ratio ∗ version ∗ group	Does the effect of task version on EV ratio differ in the GD group compared to controls?

Note. The outcome variable was the probability of choosing the high probability prospect. For models 1–5, each model was run separately for the gain and loss version of the task. EV = expected value, GD = gambling disorder, GRCS = gambling related cognitions scale, PGSI = problem gambling severity index.

^aWe performed isometric log ratio transformations on probability and magnitude pairs from each prospect, which yielded a single value representing the prospect pair's probability and a single value representing its magnitude.

^bTo directly observe EV ratio ∗ previous feedback in GD, the baseline was reversed for group, so that GD = 0.

^cBecause we predicted opposite effects of EV ratio on choice in the gain and loss versions, the dependent variable was reversed (probability of choosing the low probability prospect) in the loss version for model 6.