Both studies have limitations. Walgreens is a large chain but handles less than 15% of the Medicare market, which raises questions about the generalizability of the results. Second, neither study has a record of prescriptions filled at other pharmacies. Most Part D plans have pharmacy networks made up of preferred providers. Because persons eligible for Medicare and Medicaid (dual eligibles) and other low-income subsidy beneficiaries are permitted to change plans monthly, any research strategy that tracks prescription claims from a single company is bound to underestimate total use. Finally, because neither study had access to actual Part D enrollment data, they cannot track month-to-month changes in a person's eligibility for Part D. Therefore, they cannot distinguish among alternative explanations for months in which a person has no pharmacy claims, which could represent nonuse by an eligible Part D enrollee, loss of enrollment (perhaps because of failure to pay required premiums), or death. Each of these explanations has a different effect on estimates of average, per-person drug use, and out-of-pocket expenses.