The World Health Organization (WHO) endorses the use of population-based prevalence surveys for estimating the prevalence of trachoma. In general, the prevalence of TF in children aged 1–9 years and the prevalence of TT in adults aged ≥ 15 years are measured at the same time in any district being surveyed. This was the approach of the Global Trachoma Mapping Project, which undertook baseline surveys in > 1500 districts worldwide in order to provide the data required to start interventions where needed.<br>
The survey design recommended by WHO is a two-stage cluster random sample survey, which uses probability proportional to size sampling to select 20–30 villages, and random, systematic or quasi-random sampling to select 25–30 households in each of those villages. In most surveys, everyone aged ≥ 1 year living in selected households is examined.