An increasingly influential form of deliberative democracy centers on citizens' assemblies, where randomly selected people discuss policy questions. The legitimacy of these panels hinges on their representation of the broader population, but panelists often drop out, leading to an unbalanced composition. Although participant attrition is mitigated in practice by alternates, their selection is not taken into account by existing methods. To address this gap, we introduce an optimization framework for alternate selection. Our algorithmic approach, which leverages learning-theoretic machinery, estimates dropout probabilities using historical data and selects alternates to minimize expected misrepresentation. We establish theoretical guarantees for our approach, including worst-case bounds on sample complexity (with implications for computational efficiency) and on loss when panelists' probabilities of dropping out are mis-estimated. Empirical evaluation using real-world data demonstrates that, compared to the status quo, our method significantly improves representation while requiring fewer alternates.
