title: Quantifying cannabis use with the timeline followback approach: a psychometric evaluation.
abstract: BACKGROUND
  In the absence of a standardized cannabis unit, self-report instruments are inadequate for accurate quantification of cannabis use. The study extends the feasibility of using a cannabis substitute to reliably and validly measure quantity of cannabis use.
  
  
  METHODS
  Ninety-eight adult Australian cannabis users (M age=27.98, SD=11.10; 65.31% male) completed a 90-day Timeline Followback interview regarding their cannabis use, utilizing the cannabis substitute Marijuanilla to report on quantity of use. Ninety-two of these individuals completed the interview at two time-points, and 56 of these participants had collaterals corroborate their cannabis use reports.
  
  
  RESULTS
  Inter-rater reliability was excellent, while test-retest reliability was good to excellent. Intra-class correlation coefficients between participant and collateral reports, while similar to previous research, were unacceptable. Quantity of cannabis use statistically significantly added to frequency of use in predicting cannabis problems and dependence severity. Concurrent and discriminant validity were established with single-item and positive impression management measures, respectively. In addition, Marijuanilla appeared similar to one specimen of street seized cannabis, but not to two others. Importantly, participants' cravings to use cannabis did not increase as a result of using the cannabis substitute to report on their cannabis use.
  
  
  CONCLUSIONS
  These data suggest that utilizing Marijuanilla to facilitate the reporting of grams of cannabis use may be reliable and valid; however, such comprehensive assessment may only be necessary for clinical trials and epidemiological studies, which rely on precise estimates of cannabis use.
authors.affiliation: National Cannabis Prevention and Information Centre, University of New South Wales, Sydney, Randwick, Australia. m.norberg@unsw.edu.au
authors.forename: Melissa M
authors.initials: MM
authors.lastname: Norberg
authors.forename: Jennifer
authors.initials: J
authors.lastname: Mackenzie
authors.forename: Jan
authors.initials: J
authors.lastname: Copeland
mesh_terms: D000328:Adult; D001315:Australia; D005260:Female; D006801:Humans; D008297:Male; D008385:Marijuana Smoking / Q000453:epidemiology*; D011594:Psychometrics; D015203:Reproducibility of Results; D057566:Self Report