A meta-analysis of the crash risk of cannabis-positive drivers in culpability studies - Avoiding interpretational bias
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Background: Culpability studies, a common study design in the cannabis crash risk literature, typically report odds-ratios (OR) indicating the raised risks of a culpable accident. This parameter is of unclear policy relevance, and is frequently misinterpreted as an estimate of the increased crash risk, a practice that introduces a substantial “interpretational bias”. Methods: A Bayesian statistical model for culpability study counts is developed to provide inference for both culpable and total crash risks, with a hierarchical effect specification to allow for meta-analysis across studies with potentially heterogeneous risk parameter values. The model is assessed in a bootstrap study and applied to data from 13 published culpability studies. Results: The model outperforms the culpability OR in bootstrap analyses. Used on actual study data, the average increase in crash risk is estimated at 1.28 (1.16–1.40). The pooled increased risk of a culpable crash is estimated as 1.42 (95% credibility interval 1.11–1.75), which is similar to pooled estimates using traditional ORs (1.46, 95% CI: 1.24–1.72). The attributable risk fraction of cannabis impaired driving is estimated to lie below 2% for all but two of the included studies. Conclusions: Culpability ORs exaggerate risk increases and parameter uncertainty when misinterpreted as total crash ORs. The increased crash risk associated with THC-positive drivers in culpability studies is low.
Culpability studyMeta-analysisCannabisCrash risksBayesian inference
Project:Oppdragsgiver: Norges forskningsråd
Oppdragsgivers prosjektnr.: 240235
Frisch prosjekt: 4143 - Values, beliefs and policy options: Beyond prevalence-centric prohibitions