Randomized Controlled Trial Definition

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Patient and doctor shaking hands. Walker and Walker / DigitalVision / Getty Images

Definition of a Randomized Controlled Trial

A randomized controlled trial is a type of experimental study where people are randomly assigned to either a control or intervention group. Then, something is done to the intervention group (they are given drugs, educational seminars, counseling, etc.), while the control group is left alone or given a placebo. After the intervention, scientists see if outcomes are different between the two groups.

Although the two terms are often used interchangeably, strictly speaking not every randomized trial is a randomized controlled trial. For a randomized trial to be a randomized controlled trial it has to have a control group that receives no drug or intervention. If the two groups are both assigned to different interventions, then purists do not always consider it to be "controlled" trial. However, sometimes the standard treatment is given to the control group.

When trying to determine if a drug or intervention is effective at treating or preventing a disease, a randomized controlled trial is often considered to be the gold standard. That is because with a randomized trial, unlike with an observational study, you can actually get a good idea of causation. In a randomized controlled trial, the only thing different between two groups is whether they received the drug. Therefore, if people who got the drug had better outcomes, there's a reasonable chance that the drug caused the outcomes.

In other types of studies, subtle but important differences are harder to control.

Even though a randomized controlled trial might be considered the "best" way to study a problem, such studies are not always practical or ethical. Randomized controlled trials may also, sometimes, simply not be an appropriate way to address a given problem.

It's important to remember that just because a study isn't a randomized controlled trial doesn't mean it's a bad study or a useless study. Instead, every study must be judged on its own merits to see how much weight its results should be given. Not even all randomized controlled trials are well designed.

Examples of Randomized Controlled Trials

During a randomized controlled trial of an HIV vaccine, they might start with a group of 1000 people, 500 would receive the vaccine while 500 would receive a placebo shot, and then scientists would see how many people in each group became HIV positive over time.

A randomized controlled trial of treatment as prevention (TaSP) might giving the HIV positive person in a serodiscordant couple early HIV treatment. Then the control group would receive standard treatmentĀ  (waiting to start cART, recommending condoms). Such studies were used to show that TaSP is an effective way to reduce sexual transmission of HIV.


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Paquette D, Schanzer D, Guo H, Gale-Rowe M, Wong T. The impact of HIV treatment as prevention in the presence of other prevention strategies: lessons learned from a review of mathematical models set in resource-rich countries. Prev Med. 2014 Jan;58:1-8. doi: 10.1016/j.ypmed.2013.10.002.

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