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): |
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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 |