Efficient quantile covariate adjusted response adaptive experiments

In program evaluation studies, understanding the heterogeneous distributional impacts of a program beyond the average effect is crucial. Quantile treatment effect (QTE) provides a natural measure to capture such heterogeneity. While much of the existing work for estimating QTE has focused on analyzing observational data based on untestable causal assumptions, little work has gone into designing randomized experiments specifically for estimating QTE. In this manuscript, we propose two covariate-adjusted response adaptive design strategies–fully adaptive designs and multi-stage designs–to efficiently estimate the QTE. We demonstrate that the QTE estimator obtained from our designs attains the optimal variance lower bound from a semiparametric theory perspective, which does not impose any parametric assumptions on underlying data distributions. Moreover, we show that using continuous covariates in multi-stage designs can improve the precision of the estimated QTE compared to the classical fully adaptive setting. We illustrate the finite-sample performance of our designs through Monte Carlo experiments and one synthetic case study on charitable giving. Our proposed designs offer a new approach to conducting randomized experiments to estimate QTE, which can have important implications for policy and program evaluation.