The Hierarchy's Peak
Randomised controlled trials (RCTs) represent the highest standard of evidence for establishing intervention efficacy. Randomisation—randomly assigning subjects to treated or control groups—eliminates selection bias and equalises treated and control groups on all variables except treatment. This fundamental principle enables causal inference impossible in observational studies.
Randomisation methods vary. Simple randomisation flips a coin for each subject. Block randomisation ensures balanced group sizes throughout recruitment. Stratified randomisation balances subpopulations. Minimisation algorithms adaptively randomise to balance prognostic covariates.
Allocation concealment—ensuring those enrolling subjects don't know forthcoming assignments—prevents unconscious bias in subject selection. Blinding prevents bias from knowledge of group assignment. Placebo selection requires attention: placebo should be indistinguishable from active treatment.
Crossover designs enable within-subject comparisons, reducing variability. Pragmatic versus explanatory trial designs represent different objectives. Explanatory trials maximize internal validity through highly selected participants and controlled conditions. Pragmatic trials maximize external validity through heterogeneous real-world populations.
RCT limitations restrict applicability. Cost constraints prevent trials of low-cost interventions. Ethical constraints prevent randomising to harmful treatments. Duration constraints prevent investigating long-term effects of lifelong patterns. Rare outcomes sometimes make trials impractical.