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.