What Is a Randomised Controlled Trial?
A randomised controlled trial (RCT) is an experimental evaluation design that randomly assigns participants, communities, or units to either a treatment group (receiving the intervention) or a control group (not receiving it, or receiving a placebo/alternative). Random assignment makes the groups comparable in expectation on observed and unobserved characteristics, though realized baseline differences can remain. Follow-up differences can estimate the effect of assignment under the design's assumptions. Differential attrition, spillovers, noncompliance, measurement differences, or implementation failures can threaten interpretation; randomization alone does not remove them.
RCTs are considered the gold standard for impact evaluation when ethical and practical constraints allow. They directly address the counterfactual question - "what would have happened without the intervention?" - by using the control group as a proxy for this unobservable scenario.
Why It Matters
Well-conducted RCTs can provide strong internally valid evidence about the effect of assignment in the study context. In an M&E field often dominated by pre-post comparisons that cannot rule out alternative explanations, RCTs offer strong causal evidence when assignment, implementation, measurement, and analysis support it. This matters particularly when:
- Scale-up decisions depend on estimating benefits, harms, uncertainty, and applicability
- Agreed evaluation questions call for an experimental design where ethical and practical
- Resource allocation between competing interventions requires comparative effectiveness data
- Policy decisions hinge on knowing whether an approach causes improvement
The ability to claim with confidence that "this intervention caused those outcomes" transforms RCTs from academic exercises into powerful tools for evidence-based decision-making in international development.
In Practice
RCTs appear in M&E work primarily as impact evaluations conducted after a program has demonstrated initial feasibility. The design requires:
- Random assignment mechanism: typically using random number generators or lottery systems to allocate participants to treatment and control groups
- Design-specific baseline data: often useful for precision and understanding the sample. A valid post-test randomized comparison does not logically require baseline outcome measurements, and balance tests cannot prove that randomization succeeded
- Implementation fidelity monitoring: ensuring the treatment group actually receives the intervention as designed while the control group does not
- Follow-up measurement: collecting outcome data after the intervention period to compare groups
- Statistical analysis: estimate the prespecified treatment contrast with effect sizes, confidence intervals, and appropriate handling of clustering, attrition, and noncompliance
Common applications include testing educational interventions (e.g., does a new teaching method improve learning?), health programs (e.g., does a vaccination campaign reduce disease incidence?), and economic development initiatives (e.g., does microfinance access increase household income?).
Ethical considerations are critical: RCTs may be inappropriate when withholding an intervention from a control group would cause harm, when the intervention is already proven effective elsewhere, or when random assignment is politically infeasible. In such cases, quasi-experimental designs offer an alternative whose credibility depends on the assignment context and identifying assumptions.
Method Source
J-PAL, The elements of a randomized evaluation, threats to internal validity. This is methodological guidance, not a funding requirement.
Last updated October 15, 2026First published March 3, 2026
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RCT, Randomized Controlled Trial, Experimental Design