Objective: The objectives of this study are to describe and define a new adherence measure, the Daily Possession Ratio (DPR), and to contrast that measure with two variants of the Medication Possession Ratio (MPR, truncated MPR). Methods: This study was a retrospective analysis of the North Carolina Medicaid administrative claims data from July 1999 to June 2000. Data for non-HMO, non-hospitalized, non-pregnant schizophrenia patients (ICD-9-CM=295.**) with at least one antipsychotic were aggregated to the person-quarter level. The daily possession ratio was defined as the number of days one or more antipsychotics was available divided by the total days in the quarter. Adherence rates were also estimated for subjects that switched medications or had therapeutic duplication in the quarter. Results: The final sample consisted of 25,200 person-quarters from 7,069 individuals. For person quarters with single antipsychotic use, adherence to antipsychotics as a class was: DPR=0.607; truncated MPR=0.640; MPR= 0.695 (p<0.0001). For person quarters with therapeutic duplication, the following adherence measures were observed: DPR = 0.669; truncated MPR=0.774; MPR= 1.238 (p<0.0001). Conclusion: The DPR provides a more conservative estimate of adherence than the MPR across all type of users, however the differences between the two methods are more substantial for persons switching therapy and prescribed therapeutic duplication, where MPR may overstate true adherence. DPR should be considered when a measure of adherence to a class of medications is sought, particularly in clinical situations where multiple medications within a class are often used concurrently.