level,label,short_description,design_examples,causal_inference,typical_uses
1,Weakest,"Outcomes measured at one point in time, or before-and-after with no comparison group","Cross-sectional survey; pre-post case study","No basis for causal inference","Programme description, hypothesis generation"
2,Weak,"Outcomes measured before and after intervention with a non-equivalent comparison group","Pre-post with comparator; matched cohort without rigorous matching","Limited; selection bias and time effects confounded","Early-stage programme review, formative evaluation"
3,Moderate,"Outcomes measured in multiple treatment and comparison units, with comparison units selected to be similar","Multi-site comparison; basic propensity-score matching; cohort study with covariate adjustment","Plausible if comparison group is well-matched","Mid-stage evaluation when randomisation is infeasible"
4,Strong,"Comparison between treatment and comparison units accounting for unobservable differences","Difference-in-differences; regression discontinuity; instrumental variables; interrupted time series; synthetic control","Strong if identifying assumptions hold","Quasi-experimental impact evaluation"
5,Strongest,"Random assignment of units to treatment and control conditions","Randomised controlled trial; cluster RCT; stepped-wedge RCT","Highest internal validity available","Pilot evaluation, programme that can be randomised"
