Cluster randomised trial — DTM&H MCQ

Correct answer: C — Cluster randomised trial

The correct answer is C, cluster randomised trial. When the unit of allocation is the village rather than the individual, the study design is by definition a cluster randomised trial: whole clusters (villages) are randomised to intervention (insecticide-treated nets) or control, which is standard when the intervention acts at a community level and individual randomisation would risk contamination between exposed and unexposed households sharing mosquito populations. This design also allows estimation of community-level (herd) protection from reduced vector density, which individual randomisation cannot capture, and requires analysis methods that account for within-cluster correlation (e.g. generalised estimating equations or mixed models). Why the other options are wrong: E. Case-control study: this is an observational design comparing malaria cases with disease-free controls to look back at exposure differences; it does not involve prospective randomisation of any unit and cannot be used to test an intervention's effect this way. A. Ecological time trend: this compares aggregate outcomes across time or regions without individual or cluster-level randomisation, so it cannot establish causation as robustly and is highly vulnerable to confounding. B. Diagnostic accuracy study: this evaluates how well a test identifies disease against a reference standard; it has no role in assessing whether an intervention reduces incidence. D. Cross-sectional survey: this measures prevalence at a single time point and cannot assess incidence or the causal effect of an intervention introduced through randomisation. Key point: randomising groups (villages) rather than individuals to an intervention that could contaminate neighbours defines a cluster randomised trial, the design of choice for community-level interventions such as insecticide-treated nets.

Reference: London School of Hygiene & Tropical Medicine / PLOS Medicine: West PA et al., "Indoor Residual Spraying in Combination with Insecticide-Treated Nets Compared to Insecticide-Treated Nets Alone for Protection against Malaria: A Cluster Randomised Trial in Tanzania", PLOS Medicine 2014 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3988001/); methodology consistent with MRC/NIHR guidance on cluster randomised trial design.