Does Part D abet advantageous selection in Medicare Advantage?

The use of risk-adjustment formulae in setting payments to Medicare Advantage (MA) plans reduces the potential for advantageous selection on factors included in the formulae, but can theoretically worsen overall selection if plans are able to target beneficiaries based on excluded factors. Since MA medical risk-adjustment excludes prescription drug utilization, demand for drugs can be exploited by plans to induce advantageous selection. We show evidence that the introduction of Medicare Part D provided a mechanism for MA plans to increase selection, and that consumers responded, increasing MA market shares among beneficiaries taking drugs associated with the strongest advantageous selection incentives. For the average Medicare beneficiary in our sample, we estimate that this change in advantageous selection following the introduction of Medicare Part D increased the probability of enrolling in an MA plan by about 7.1%.