Validation of self-reported cannabis dose and potency an ecological study.
To assess the reliability and validity of self-reported cannabis dose and potency measures. Cross-sectional study comparing self-reports with objective measures of amount of cannabis and delta-9-tetrahydrocannabinol (THC) concentration. Ecological study with assessments at participants homes or in a coffee shop. Young adult frequent cannabis users (n 106) from the Dutch Cannabis Dependence (CanDep) study. The objectively measured amount of cannabis per joint (dose in grams) was compared with self-reported estimates using a prompt card and average number of joints made from 1 g of cannabis. In addition, objectively assessed THC concentration in the participants cannabis was compared with self-reported level of intoxication, subjective estimate of cannabis potency and price per gram of cannabis. Objective estimates of doses per joint (0.07-0.88 gjoint) and cannabis potency (1.1-24.7%) varied widely. Self-reported measures of dose were imprecise, but at group level, average dose per joint was estimated accurately with the number of joints made from 1 g limit of agreement (LOA) -0.02 g, 95% confidence interval (CI) -0.29 0.26, whereas the prompt card resulted in serious underestimation (LOA 0.14 g, 95% CI -0.10 0.37). THC concentration in cannabis was associated with subjective potency average 3.77% (P 0.002) and (very) strong 5.13% more THC (P < 0.001) than (very) mild cannabis and with cannabis price (about 1% increase in THC concentration per euro spent on 1 g of cannabis, P < 0.001), but not with level of intoxication. Self-report measures relating to cannabis use appear at best to be associated weakly with objective measures. Of the self-report measures, number of joints per gram, cannabis price and subjective potency have at least some validity.