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To the Editor: Dr Joyce and colleagues¹ used a cross-sectional design, in which there were different participants at each cost-sharing level and in each year. This approach is significantly less rigorous than longitudinal approaches that study temporal changes in behavior with concurrent controls.^2-3 The use of the essentially cross-sectional design in this study is particularly concerning because of potential selection biases from health plan adverse selection; ie, when patients or employers with greater anticipated health expenditures deliberately choose plans with lower levels of cost sharing, and vice-versa. Such biases would amplify any expected cost differences between plans with varying levels of cost sharing. As the authors acknowledge, the lack of drug plan choice among employees of employers in this study provides only limited protection for this type of selection bias.

Another concern is the use of prescription drug costs as the primary outcome. The changes in drug costs across levels . . . [Full Text of this Article]

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