The effect of heterogeneous variance on efficiency and power of cluster randomized trials with a balanced 2x2 factorial design

Sample size calculation for cluster randomized trials (CRTs) with a 2x2 factorial design is complicated due to the combination of nesting (of individuals within clusters) with crossing (of two treatments). Typically, clusters and individuals are allocated across treatment conditions in a balanced fashion, which is optimal under homogeneity of variance. However, the variance is likely to be heterogeneous if there is a treatment effect. An unbalanced allocation is then more efficient, but impractical because the optimal allocation depends on the unknown variances. Focusing on CRTs with a 2x2 design, this paper addresses two questions: How much efficiency is lost by having a balanced design when the outcome variance is heterogeneous? How large must the sample size be for a balanced allocation to have sufficient power under heterogeneity of variance? We consider different scenarios of heterogeneous variance. Within each scenario, we determine the relative efficiency of a balanced design, as a function of the level (cluster, individual, both) and amount of heterogeneity of the variance. We then provide a simple correction of the sample size for the loss of power due to heterogeneity of variance when a balanced allocation is used. The theory is illustrated with an example of a published 2x2 CRT

Assessing the Efficacy of MOTI-4 for Reducing the Use of Cannabis Among Youth in the Netherlands: A Randomized Controlled Trial

The Moti-4 intervention, in which motivational interviewing, self-monitoring, and strengthening behavioral control are used, was developed in the Netherlands in response to several rapid assessments of problematic use of cannabis among vulnerable adolescents. The main goal of the study reported in this article was to determine whether the Moti-4 intervention was able to reduce two outcome measures pertaining to the level of cannabis use; the amount of Euros spent a week on cannabis and the mean number of cannabis joints (cigarettes) smoked in a week.In a randomized controlled trial (RCT) with a 6-month follow-up, 27 trained Dutch prevention workers recruited 71 Moti-4 participants and 60 controls assigned to usual care. Participants were Dutch youth aged 14-24 years who had used cannabis during the preceding month. At baseline (T0), post-test (T1) and 6-month follow-up (T2), participants completed a questionnaire with 51 items. The 27 prevention workers also completed a checklist to assess the fidelity of delivering each item to each participant in the Moti-4 protocol. Multilevel and binary logistic regression was used to assess the impact of the prevention worker and 14 participant variables on the likelihood of drop-out. Mean scores for cannabis use outcome measures by Moti-4 participants and controls at baseline, T1 and T2 were compared using paired sample t-tests. Top-down multiple regression was used to assess relationships between Moti-4 and 13 other variables on the one hand and changes in weekly cannabis use at T1 and T2 on the other. The Moti-4 experimental condition had a significant and positive influence in reducing the level of expenditure on cannabis (p < 0.05). There was no significant difference in outcome, neither for the 4 participating institutes nor for the professionals implementing the intervention. Baseline cannabis use was the strongest predictor (p < 0.001) of weekly cannabis expenditure at posttest and 6-month follow-up. This effect was still present at T2. Being female, having two Dutch parents and perceived behavioral control also made significant positive contributions (p < 0.05). Attitude at baseline was only related to cannabis expenditure after 6 months (p = 0.005). At T2 Moti-4 participants were found to have a significant reduction in the number of joints smoked weekly compared to T0 (on the average 4 joints). The study demonstrated that Moti-4 is an effective intervention to reduce cannabis use in youth

Probabilistic feature models for unfolding

Contains fulltext : mmubn000001_130535257.pdf (publisher's version ) (Open Access)Promotor : E. Roskam156 p