Minimum detectable effect size for main outcomes (accounting for sample
design and clustering)
The activities to evaluate the Trust Youth program described in this protocol are part of a multistage research, evaluation, and program development process, consistent with the Medical Research Council (MRC) Framework for Developing and Evaluating Complex Interventions (Craig et al., 2008). Consistent with the MRC Framework, we take a developmental approach to evaluation. In this project stage, the project focuses on pilot testing the program, evaluating its processes and implementation to inform potential program improvements, and gaining better information on the direction and magnitude of effects to inform future sample size calculations for a more fully powered evaluation in the future. In alignment with the MRC Framework, this initial trial will help to pilot andd refine sampling and randomization procedures in advance of a second round of recruitment, explore the direction and magnitude of likely program effects, and help test/inform sample size calculations and outcome measures for a better-powered impact study before the second round of recruitment begins to achieve a fully powered study.
While this preliminary randomized trial of Trust Youth is intentionally not fully powered to estimate statistically significant effects of modest sizes or by subgroups, this sample size is sufficient to estimate overall treatment effects if we find a similar effect size (34% mean difference) for housing stability that was observed from a randomized trial of a “Housing First” rental subsidy and wraparound services intervention for young adults with mental illness in Canada (Kozloff et al., 2016). This trial is the closest comparison in the literature available from which to draw an effect size assumption and used the same primary outcome measure of housing stability that we plan to use. We assume 95% confidence intervals, two-sided tests, 80% power, 20% attrition, and SD = 0.43.
Using the same assumptions, a sample size of 130 (60 in Trust Youth [TY] program and 70 in services-as-usual [SAU]) would be sufficient to demonstrate statistically significant results with a 19% mean difference. A larger sample size also provides a better buffer in the event of higher levels of evaluation dropout, missing data, or unexpected levels of variation between outcomes.
REFERENCES
Craig, P., Dieppe, P., Macintyre, S., Michie, S., Nazareth, I., & Petticrew, M. (2008). Developing and evaluating complex interventions: the new Medical Research Council guidance. BMJ, 337.
Kozloff, N., Adair, C. E., Lazgare, L. I. P., Poremski, D., Cheung, A. H., Sandu, R., & Stergiopoulos, V. (2016). " Housing first" for homeless youth with mental illness. Pediatrics, 138(4), e20161514.