An intervention trial tells us more than an observational study . In an intervention trial, researchers "intervene" in the participants' lives in some way to see what happens. This type of study is prospective—it follows people into the future.

In a chemopreventionintervention trial, for example, participants take a specific agent, such as a nutritional supplement, thought to help prevent a particular disease. They are then followed to see if they are less likely to get the disease than others not taking the agent. The best type of intervention trial uses a control group receiving a placebo (inactive pill), randomly assigns participants to the treatment and control groups, and uses the double-blind format in which neither researchers nor subjects know which treatment each participant receives. Intervention trials usually focus on high-risk groups of people rather than the population at large.

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