This study addresses a fundamental challenge in the empirical application of the Capability Approach: the measurement of the “capability set” as an opportunity set. Unlike standard utility-based measures, measuring capability requires assessing the welfare of potential activities—including those not chosen. We propose a novel methodology that bridges normative social choice theory and econometric causal inference. Specifically, we interpret the Average Treatment Effect derived from panel data fixed-effects models as capturing counterfactual welfare differences between alternative actions. Using a unique panel dataset of elderly individuals in Japan, focusing on “going-out” versus “staying-home” behavior, we evaluate the size of capability sets and the degree of “unfreedom” as the welfare gap between actions. Furthermore, we propose and apply several aggregation rules, ranging from Utilitarian to Rawlsian, to construct group-level capability measures. Our empirical results demonstrate that the ranking of social groups varies significantly depending on the normative aggregation rule employed, highlighting the importance of explicitly defining the informational basis of social evaluation.