Categorical and Continuous Features in Counterfactual Explanations of AI Systems

Recently, eXplainable AI (XAI) research has focused on the use of counterfactual explanations to address interpretability, algorithmic recourse, and bias in AI system decision-making. The proponents of these algorithms claim they meet users' requirements for counterfactual explanations. For instance, many claim that the output of their algorithms work as explanations because they prioritise "plausible", "actionable" or "causally important" features in their generated counterfactuals. However, very few of these claims have been tested in controlled psychological studies, and we know very little about which aspects of counterfactual explanations help users to understand AI system decisions. Furthermore, we do not know whether counterfactual explanations are an advance on more traditional causal explanations that have a much longer history in AI (in explaining expert systems and decision trees). Accordingly, we carried out two user studies to (i) test a fundamental distinction in feature-types, between categorical and continuous features, and (ii) compare the relative effectiveness of counterfactual and causal explanations. The studies used a simulated, automated decision-making app that determined safe driving limits after drinking alcohol, based on predicted blood alcohol content, and user responses were measured objectively (users' predictive accuracy) and subjectively (users' satisfaction and trust judgments). Study 1 (N=127) showed that users understand explanations referring to categorical features more readily than those referring to continuous features. It also discovered a dissociation between objective and subjective measures: counterfactual explanations elicited higher accuracy of predictions than no-explanation control descriptions but no higher accuracy than causal explanations, yet counterfactual explanations elicited greater satisfaction and trust judgments than causal explanations. Study 2 (N=211) found that users were more accurate for categorically-transformed features compared to continuous ones, and also replicated the results of Study 1. The findings delineate important boundary conditions for current and future counterfactual explanation methods in XAI.