Randomized controlled trials and challenge trials: design and criterion for validity.

This article is the third of six articles addressing systematic reviews in animal agriculture and veterinary medicine. This article provides an overview of clinical trials, both randomized controlled trials (RCTs) and challenge trials, where the disease outcome is deliberately induced by the investigator. RCTs are not the only study design used in systematic reviews, but are preferred when available as the gold standard for evaluating interventions under real-world conditions. RCTs are planned experiments, which involve diseased or at-risk study subjects and are designed to evaluate interventions (therapeutic treatments or preventive strategies, including antibiotics, vaccines, management practices, dietary changes, management changes or lifestyle changes). Key components of the RCT are the use of one or more comparison (control) groups and investigator control over intervention allocation. Important design features in RCTs include as follows: how the population is selected, approach to allocation of intervention and control group subjects, how allocation is concealed prior to enrolment of study subjects, how outcomes are defined, how allocation to group is concealed (blinding) and how withdrawals from the study are managed. Guidelines for reporting important features of RCTs have been published and are useful tools for writing, reviewing and reading reports of RCTs.