Clinical trials use scientific methods to evaluate the effectiveness and safety of treatments or other interventions. Trials should have both internal and external validity, and a well-conducted randomised controlled trial is considered to be the most powerful tool for evaluating interventions. Systematic error (bias) and random error could threaten the internal validity of trials, and all efforts should be made to minimise these in the design, conduct, and analysis of studies. Careful attention should be paid to issues such as randomisation, allocation concealment, blinding, and sample size. In an internally valid trial, external validity refers to the ability of the results to be generalised to the “real world” population. Issues to consider in determining the external validity of a study include the setting of the trial, the study population, the types of interventions used, duration of follow-up, and the types of outcome and how they were assessed.