Randomized Controlled Trials

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A comparative controlled trial is designed to assess the relative effectiveness of two or more treatments (exposures). Because investigators strive to identify the better treatment if one exists and to convince others of the validity of the results, they need to design and conduct trials carefully to minimize bias.
Until relatively recently in medical and public health history, investigators rarely used formally controlled experimentation in designing trials. During the twentieth century, the controlled clinical trial has gained increasing recognition as the best approach to evaluating health care and prevention alternatives. A relatively recent development has been the use of randomization in the assignment of participants to comparison groups.
R. A. Fisher developed randomization as a basic principle of experimental design in the 1920s. The successful adaptation of randomized controlled trials (RCTs) to health care took place in the late 1940s, largely because of the advocacy and developmental work of Sir Austin Bradford Hill while at the London School of Hygiene and Tropical Medicine.1 His efforts resulted in the first use of random numbers to allocate trial participants.2
The random allocation of trial participants persists to this day as the most crucial aspect of the design of a controlled trial.3 Unfortunately, it is also perhaps the least understood aspect. Investigators poorly comprehend the rationale and mechanisms behind random allocation. In this article, I focus my attention on the allocation process. I discuss the important elements of randomization, present examples of the deciphering of assignment sequences before allocation, and provide a few methodologic recommendations. Although focusing on randomization, I also discuss the need for improved reporting of all aspects of RCTs.
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