Query to the author of “Spectacle compliance amongst rural secondary school children in Pune district, India”

We could not understand the reason for assuming an absolute precision of exactly 4% (and not 2, 5 or 10%) for a one time observation of a fact (wearing spectacles) with a binary outcome (yes‐no) and binomial distribution, when no previous studies stating variability on the subject are available from the area. If a meta‐analysis has been used it should be discussed rather than leaving for assumption and confusion in readers’ minds. Design effect used is dependent on the rho and in turn on the intra‐cluster and inter‐cluster variability. Since, no previous studies from same area are available, how was this variability determined by authors? Design effect and sample size figures need to be interpreted with caution, because number of clusters is less than 30 and cluster size is large. Sample size was calculated with a compliance rate of 35% t.[1] Previous studies give rates of compliance at 19.5% from rural central India,[2] 13.4% from Mexico,[3] 30% from Baltimore[4] and 37.7% from rural China.[5] Maybe the rural central India figures would have been more appropriate.