The utility of routinely collected data in evaluating important policy changes: the New Zealand alcohol purchasing age limit example.

We used the recent lowering of the alcohol purchasing age in New Zealand to examine the proposition that routinely collected data are often insufficient in evaluating important policy changes. We estimated prechange and postchange incidence rate ratios for actual and hypothetical population sizes and hospital admissions related to alcohol poisoning and assaults. Even with a hypothetical youth population 10 times larger than New Zealand's actual youth population, comparisons were underpowered because there were too few observations. Governments should use the enactment of health legislation as an opportunity to build the research evidence base by ensuring that evaluations are initiated in advance.