Minimum detectable effect size for main outcomes (accounting for sample
design and clustering)
Research Question 7
With 30 clusters, n = 21, power = .8, α (twosided) = .05, ICC = .05 (based on earlier unpublished work using GPA as outcome measure), the expected minimal detectable effect is f = 0.327 (ratio of the treatment main effect to the total standard deviation, Zhang & Yuan, 2018. Lacking comparable studies, we estimate this as medium to large (Cohen, 1977)). We test the hypotheses for this question two-sided as we formulate no expectation of a direction.
Research Questions 8 - 9
For the survey, we expect between 30% response and 80% response. In the case of 30% response, 30 clusters, n = 7, power = .8, α(one-sided) = .05, ICC = .05, the minimal detectable effect is f = .401 (large, Cohen 1977). In the case of 80% response, 30 clusters, n = 17, power = .8, α(one-sided) = .05, ICC = .05, the minimal detectable effect is f = .303 (medium to large, Cohen, 1977).
According to these calculations, we need substantial effect sizes to reach significant outcomes. This setting does not allow for higher number of participants, but we know of no preceding research in this setting or future opportunity to answers our questions. We do expect to substantially limit standard errors by adding relevant available predictor variables.
Cohen, J. (1977). Statistical Power Analysis for the Behavioral Sciences (Revised Edition). Academic Press.