“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”.

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.”

https://www.ncbi.nlm.nih.gov/pubmed/30468948

https://www.sciencedirect.com/science/article/pii/S0001457518304706?via%3Dihub