Sample sizes in cluster surveys must be greater than those in surveys using simple random sampling to provide similarly precise prevalence estimates, because results from subjects examined in the same cluster cannot be assumed to be independent. Therefore, a crucial aspect of cluster sampling is estimation of the intracluster correlation coefficient (ICC): the degree of relatedness of outcomes in a given cluster, defined as the proportion of total variance accounted for by between-cluster variation. The ICC is used to calculate the design effect (DE) for sample size calculations.
This study used a parametric bootstrap model to estimate ICC for trachomatous inflammation-follicular (TF) in 1-9-year-olds, from 261 population-based trachoma prevalence surveys completed using standardised GTMP methodologies in Ethiopia, Mozambique, and Nigeria from 2012-2016.
Results from this study were used to draw up the most recent WHO 2018 recommendations on DE estimates for sample size calculations for surveys. These recommendations replace the previous WHO 2006 recommendations.