Study Design and Sample Size

The study is a non- randomized controlled trial including eight intervention and six control schools. The study assessed knowledge, attitudes and behaviors of the students three times over a period of eighteen months: March 2009 (Baseline), March 2010 (TI) and September 2010 (t20. We based the sample size calculation on the study objective of assessing whether or not the intervention influenced the time trend in condom use and recent history of sexual intercourse. Sample size calculations were conducted with Wald tests for odds ratio resulting from regression models with two binary variables (intervention/ control and TO/T1 OR T0/T2) and their interaction. For logistic regression models, a minimum of 1,241 observations are required to detect an adjusted odds ratio of 2 or more with 80% power under conservative assumptions of 30 % baseline prevalence of the outcome variable and no changes over time in the control group. For linear regression models, a minimum of 348observations are required to detect a small standardized effect size (Cohen's d) of 0.3 with 80% power at the 0.05 significance level. Further, we assumed a design effect of 2, due to possibly strong correlation of repeated measurements from the same participant (TO/TI/T2), resulting in a minimum of 2,482 observations required from 1,241 participants. Anticipating a 25% loss t follow -up. We increased the target sample size to 1,655 participants at TO. Eventually for the research, the target sample was increased from 1,655 to 1950.

Question

The investigators described their sample size calculations, which are relatively complex. Did the investigators achieve the desired sample size? Do you think that the study findings are made stronger or weaker by the size of the sample? Explain your answer.