The effect of parity-induced copayment reductions on adolescent utilization of substance use services

The purpose of this study was to determine if the reduction in copayment amount by a large self-insured state employer increased utilization of adolescent services. Specifically, the study sought to discover if the number of unique adolescent users of substance use outpatient services increased as a result of reductions in cost-sharing arrangements.

The data utilized in this study were 31,585 records from administrative claims data on utilization of mental health and substance abuse services from members of a state indemnity plan fromJuly 1998 through December 2001, translating to 36 months of pre-intervention data and 6 months of postintervention data. Monthly longitudinal data before and after benefit design change were analyzed using a quasi-experimental time series design, using Box and Jenkins' autoregressive, integrated, moving-average time-series modeling methods. The primary outcome measure was the number of unique users of services.

The hypothesis that service utilization would increase following the implementation of a reduction in copayment amount (the intervention) was supported in these analyses for adolescents' substance use service utilization. A significant increase in the number of unique adolescent users of substance use services was detected in the month following the intervention (p < .01).

The results of this study suggest that a reduction in adolescents' substance use service copayment requirements to a level equal to those for general medical services may be a step toward assuring full parity between such types of services. These findings provide potentially important information regarding the possible effects of broader policy changes, as parity in benefit design is a common component of laws that attempt to ensure "full parity."