title: Validation of self-reported cannabis dose and potency: an ecological study.
abstract: OBJECTIVE
  To assess the reliability and validity of self-reported cannabis dose and potency measures.
  
  
  METHODS
  Cross-sectional study comparing self-reports with objective measures of amount of cannabis and delta-9-tetrahydrocannabinol (THC) concentration.
  
  
  METHODS
  Ecological study with assessments at participants' homes or in a coffee shop.
  
  
  METHODS
  Young adult frequent cannabis users (n = 106) from the Dutch Cannabis Dependence (CanDep) study.
  
  
  METHODS
  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 participant's cannabis was compared with self-reported level of intoxication, subjective estimate of cannabis potency and price per gram of cannabis.
  
  
  RESULTS
  Objective estimates of doses per joint (0.07-0.88 g/joint) 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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Trimbos Institute, Netherlands Institute of Mental Health and Addiction, Utrecht, the Netherlands.
authors.forename: Peggy
authors.initials: P
authors.lastname: van der Pol
authors.forename: Nienke
authors.initials: N
authors.lastname: Liebregts
authors.forename: Ron
authors.initials: R
authors.lastname: de Graaf
authors.forename: Dirk J
authors.initials: DJ
authors.lastname: Korf
authors.forename: Wim
authors.initials: W
authors.lastname: van den Brink
authors.forename: Margriet
authors.initials: M
authors.lastname: van Laar
mesh_terms: D000328:Adult; D002188:Cannabis / Q000737:chemistry*; D003430:Cross-Sectional Studies; D013759:Dronabinol / Q000008:administration & dosage / Q000032:analysis*; D005260:Female; D006213:Hallucinogens / Q000008:administration & dosage / Q000032:analysis*; D006801:Humans; D008297:Male; D002189:Marijuana Abuse; D009426:Netherlands; D015203:Reproducibility of Results; D057566:Self Report / Q000592:standards*; D055815:Young Adult
chemical_list: D006213:Hallucinogens; D013759:Dronabinol
keywords: Dose; THC; potency; quantity; self-report; validation