Nonrandomized Trials: Designs and Methodology

Abstract The randomized controlled trial is the study design of choice in order to assess the effectiveness of an intervention. Sometimes, the nonrandomized trial study design is used, whereby participants are allocated to treatment groups using nonrandom methods. The design has the potential for confounding with the consequence that internal validity may be compromised. Nonetheless, some nonrandom methods have benefits. In preference trials, patients can choose their treatment, minimizing the effects of treatment preference that can threaten than can threaten the internal validity and the external validity. Although not a trial by definition, the before‐and‐after study can demonstrate the effectiveness of an intervention in a similar way to a nonrandomized trial. The application and methodological implications of the nonrandomized trial, preference trial, and before‐and‐after study designs are outlined. Approaches to treatment group allocation, along with the benefits and disadvantages, are discussed.