Analysis of longitudinal data to evaluate a policy change.

Longitudinal data analysis methods are powerful tools for exploring scientific questions regarding change and are well suited to evaluate the impact of a new policy. However, there are challenging aspects of policy change data that require consideration, such as defining comparison groups, separating the effect of time from that of the policy, and accounting for heterogeneity in the policy effect. We compare currently available methods to evaluate a policy change and illustrate issues specific to a policy change analysis via a case study of laws that eliminate gun-use restrictions (shall-issue laws) and firearm-related homicide. We obtain homicide rate ratios estimating the effect of enacting a shall-issue law, which vary between 0.903 and 1.101. We conclude that in a policy change analysis it is essential to select a mean model that most accurately characterizes the anticipated effect of the policy intervention, thoroughly model temporal trends, and select methods that accommodate unit-specific policy effects. We also conclude that several longitudinal data analysis methods are useful to evaluate a policy change, but not all may be appropriate in certain contexts. Analysts must carefully decide which methods are appropriate for their application and must be aware of the differences between methods to select a procedure that generates valid inference. Copyright 2008 John Wiley & Sons, Ltd.