Table 1. Summary of characteristics of trachoma survey methods.
| Characteristic | PBPS, e.g. CRS | ASTRA | TRA |
|---|---|---|---|
| Sampling design | One or multistage cluster sample | Stratified random sample from population list; modified LQAS | Convenience sample of communities with greatest perceived trachoma burden |
| Sub-populations | Clusters based on geographical or political boundaries; supposed to be heterogeneous | Lots based on geographical or political boundaries; supposed to be homogenous | Villages or communities |
| Sample size | Estimate based on a population proportion | Estimate based on hypothesis test (desired proportion and level of Type I and Type II errors) | Fixed sample of 50 children aged 1–9 years |
| Lists of units | List of primary sampling units needed; complete census not needed, but useful | Population census is essential | No census needed |
| Basis for inference | Confidence interval for estimate | Hypothesis test | Ranking of communities |
| Outcome | Overall population estimate (e.g. prevalence); estimate from individual clusters should not be calculated | Individual lots judged as acceptable or not acceptable: overall estimates if stopping rule is not used | Proportions in each village or community |
| Weighting of sample | Self-weighting if PPS | Weights calculated for each lot if overall estimate is required | Weighting not required |
| Cost | Decreased travel time and preparation; reduced cost since census not required | Low cost due to small sample sizes claimed; however, the need to sample each lot may yield higher cost for population census | Cheap since sample is convenient |
| Reasons for potential bias | Geographical clustering of sample | Small samples in each lot | Selection bias |
| Advantages | Simple and efficient to conduct; population census not required; multiple indicators may be assessed in one survey; periodic surveys allow changes in prevalence to be shown over time; multiple indicator surveys enhance interpretation of prevalence change | Small sample sizes for deciding acceptability of a lot; suitable for small study units; suitable for monitoring programme coverage; periodic surveys allow a “snap decision” on whether to continue or stop intervention | Simple and cheap to conduct. |
| Disadvantages | Does not derive estimate for individual clusters; error estimates require adjustment for sample design | Population census list essential; expertise required deciding acceptable proportions and risks; small samples in each lot may result in imprecise estimates; large sample sizes if overall estimate is required; cannot be used for multiple indicators | Inaccurate and inconsistent estimates; does not produce prevalence estimates; not based on accurate epidemiological methods; not suitable for monitoring or surveillance |
| When to use | Interest in overall population estimate; population-based prevalence surveys are the “gold standard” | Interest in information for each lot; suitable for monitoring or surveillance | May identify where prevalence surveys are required; limited use due to inadequate statistical rigour |
ASTRA, acceptance sampling trachoma rapid assessment; CRS, cluster random sampling; LQAS, lot quality-assurance sampling; PBPS, population-based prevalence surveys; PPS, probability proportional to size; TRA, trachoma rapid assessment.