Sebastian Schneeweiss M.D., S.M.
ABSTRACT
Background: Differential cost sharing (DCS) of prescription medications
is of growing importance in pharmaceutical benefits plans. However there
is little evidence of the effects on drug- and health care utiliza-tion
and even less is known about the impact on health outcomes and overall
costs.
Objective is to develop models to evaluate DCS form a policy maker’s
and a clinician’s perspective using the recent introduction of DCS for
ACE inhibitors in British Columbia as an example.
Results: Benefits plan managers are mainly interested in the overall
performance of their plan. A Policy Model tests, whether there are changes
in the time trends of outcomes after the introduction of DCS compared to
before the DCS policy. This will lead to better estimates of the overall
effect and practical consequences of the policy maker’s decision. However,
estimates are summary effects and null effects might be the average of
a benefit in non-compliers and harm in compliers. Results apply to a specific
policy implementation and are limited in their generalizability. Segmented
linear regression or generalized linear models for repeated measurements
can be used for analysis. Clinical decision maker and patients are interested
in the consequences, given their actual compliance to the policy. A Clinical
Model assesses the effects of DCS as-treated, isolated from program specific
effects such as exemption rules. However, the model must make partly unprovable
assumptions of the appropriate control of selection processes. More complex
generalized linear models controlling for selection factors must be employed.
Instrumental variable estimation should be considered to overcome the limitations
in measuring these factors.
Conclusions: Both, Policy and Clinical Models should be tested with
a clear understanding of their interpretation, using quasi experimental
time-series designs and corresponding analysis techniques to evaluate the
effects of DCS.
Keywords: Differential cost sharing, Drugs, Causal graphs, Health policy analysis, Epidemiologic methods
1) INTRODUCTION
2) BACKGROUND TO DIFFERENTIAL COST SHARING OF PRESCRIPTION MEDICATIONS
Differential Cost Sharing in the United States
Existing Evidence on the Effects of Differential
Cost Sharing
The Natural Experiment of Reference Pricing in British
Columbia as a Model for Evaluating Differential Cost Sharing
3) HYPOTHESES IN THE EVALUATION OF DCS AND THEIR INTERPRETATIONS
The Policy Model (PM):
The Clinical Model (CM):
4) UNDERSTANDING SELECTION PROCESSES USING CAUSAL GRAPHS
Measurement of Intervention and Outcomes in the
Clinical Model
Intermediate Factors that originate Policy Compliance
Patient Related Factors
Physicians Related Factors
Health System Related Factors
Unknown or Unmeasurable Factors
5) IMPLICATIONS FOR THE ANALYSIS
Policy Model: Modeling Time
Clinical Model: Controlling for Selection Factors
Clinical Model: Instrumental Variable Estimation
to control for Unmeasurable Selection Factors
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