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Table 2 Sample size determination

From: Comparing the feasibility, acceptability, clinical-, and cost-effectiveness of mental health e-screening to paper-based screening on the detection of depression, anxiety, and psychosocial risk in pregnant women: a study protocol of a randomized, parallel-group, superiority trial

Using the calculation of a confidence interval for a proportion, a 95% CI and a margin of error of 0.05: Accounting for an estimated attrition/loss to follow-up of 25% based on other reported rates in pregnant and postpartum women (55,66):
p = a priori estimate of % of interest; n = sample size n = total sample size; L = attrition rate + loss-to-follow up
n = p(1-p) [1.96/.05]2 Nnew = n/(1-L)
n = 0.85(1–0.85) [39.2]2 Nnew = 196/(1–0.25)
n = 196 Nnew =261
  1. Therefore a minimum of 261 women per group would be required.