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Table 1 Defining characteristics of cluster randomised trials (CRT), non-randomised cluster trials (NCT), controlled before-and-after study (CBA), and before-and-after trials (BA)

From: Randomised and non-randomised studies to estimate the effect of community-level public health interventions: definitions and methodological considerations

  CRT NCT CBA BA
Defining features Randomisation of adequate number of clusters to allow statistical between-arm comparison Non-random allocation of adequate number of clusters to allow statistical between arm comparison Random or non-random allocation of small number of clusters - too few to allow statistical between-arm comparison Before and after assessment of outcomes in the absence of a control group
Appropriate setting Adequate resources, randomisation feasible Adequate resources, randomisation is politically or logistically not possible Resource limited evaluations or where number of clusters is naturally constrained (e.g. a district-level intervention in a province that only has 4 districts) Mass media campaign where no unexposed group can be found
Detectable effect size given adequate sample size Small Small to moderate (depending on baseline comparability and temporal stability of outcome in control arm) Moderate to large (depending on baseline comparability and temporal stability of outcome in control arm) Large
Number of clusters At least 4–6 clusters per arm, higher if effect size is small At least 4–6 clusters per arm, higher if effect size is small At least 2 clusters per arm, unless outcome is assessed repeatedly before and after Study power is determined by number of participants and number of pre/post measures
Baseline measure of outcome of interest Not required, but may increase study power and allow adjusting for imbalances Required Required Required
Outcome assessment at multiple time-points Not required, but may increase study power Usually not required, but may increase study power Desirable, required if there is only one cluster per arm Required
Statistical analysis Direct comparison between intervention and control Direct comparison between intervention and control, by adjusting for baseline measure of outcome, by calculating change scores or by calculating the difference-in- difference Comparison before versus after; control arm only serves to demonstrate absence of trends. Analysis of slope and intercept of trends in the outcome measure (if multiple pre/post measures are available) Comparison before versus after. Analysis of slope and intercept of trends in the outcome measure (if multiple pre/post measures are available)
Special types Stepped wedge design Trials using systematic allocation (“quasi-randomised”) Controlled interrupted time series analysis Per-protocol analysis within a CRT   Interrupted time series analysis Pre/post adopter versus non-adopter analysis