A meta-analysis of interventions to reduce adolescent cannabis use
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This entry is our analysis of a review or synthesis of research findings added to the Effectiveness Bank. The original review was not published by Findings; click Title to order a copy. Free reprints may be available from the authors – click prepared e-mail. The summary conveys the findings and views expressed in the review. Below is a commentary from Drug and Alcohol Findings.

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A meta-analysis of interventions to reduce adolescent cannabis use.

Bender K., Tripodi S.J. , Sarteschi C. et al.
Research on Social Work Practice: 2011, 21(2), p. 153–164.
Unable to obtain a copy by clicking title? Try asking the author for a reprint by adapting this prepared e-mail or by writing to Dr Bender at kimberly.bender@du.edu.

The first synthesis of research on therapeutic interventions for adolescent cannabis users highlighted the relative success of family and multi-component approaches, but the evidence base was too narrow to securely determine what works best.

Summary Objective This meta-analytic A study which uses recognised procedures to combine quantitative results from several studies of the same or similar interventions to arrive at composite outcome scores. Usually undertaken to allow the intervention's effectiveness to be assessed with greater confidence than on the basis of the studies taken individually. review assesses the effectiveness of substance abuse interventions to reduce adolescent cannabis use.

Method A systematic search identified 15 randomised controlled evaluations of interventions to reduce adolescent cannabis use published between 1960 and 2008. The entire sample of youth participants was aged 12–19 and all the studies were carried out within the United States. The primary outcome variables – frequency and quantity of cannabis use – were measured between one month and a year after completion of treatment.

Results The studies and their results were varied, so outcomes were combined using an approach (a 'random effects model') which does not assume there is one 'true' impact size which simply varied by chance across the studies. Instead it assumes that impact really does differ for the different interventions and the different situations in which they were tested. These analyses resulted in roughly equivalent moderate effect sizes A standard way of expressing the magnitude of a difference (eg, between outcomes in control and intervention groups) applicable to most quantitative data. Enables different measures taken in different studies to be compared or (in meta-analyses) combined. Based on expressing the difference in the average outcomes between control and experimental groups as a proportion of how much the outcome varies across both groups. The most common statistic used to quantify this difference is called Cohen's d. Conventionally this is considered to indicate a small effect when no greater than 0.2, a medium effect when around 0.5, and a large effect when at least 0.8. The featured study instead used a variant called 'Hedges's g' which corrects for the bias that would otherwise result when few studies are available to be combined. for individual and family-based treatments, which across all the studies meant that 69% (50% is the break-even point) of young people offered the tested treatments reduced their cannabis use more than those in comparison treatments or (rarely) whose treatment was delayed. Impacts tended to wane as follow-up periods lengthened, suggesting that clinicians should when possible offer follow-up or booster sessions. The minority of studies with follow-up periods of a year or more revealed that behavioural treatment and especially Multidimensional Family Therapy had large and enduring effects.

Discussion Substance abuse treatment programmes should consider implementing evidence-based interventions highlighted in this meta-analysis which fit the needs and characteristics of their client base and agency setting. There is not yet enough published data to determine which if any of the reviewed treatments is the most effective, but evidence is strongest for family-based or multisystem approaches including Integrated Family and Cognitive Behaviour Therapy, Multidimensional Family Therapy, and Teaching Family. Given their roughly equivalent impact, the higher cost of family versus client-focused individual therapies may be a decisive consideration.

Findings logo commentary Multidimensional Family Therapy emerged as the most supported therapy in this analysis but mainly when compared to weak alternatives; advantages with respect to cognitive-behavioural therapy were minor and in the single study to test this, not statistically significant. See this Findings analysis for more on this approach and the strengths and limitations of the evidence for it being preferable to alternatives.

Last revised 01 April 2011

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