The American Cancer Society's Cancer Prevention II study began in 1982 with more than 77,000 volunteers. What type of study is this?

The study conducted by the American Cancer Society, known as the Cancer Prevention II study, is classified as a cohort study. This type of study involves following a group of individuals who are initially free of the disease in question—in this case, cancer—over a period of time to observe how various risk factors affect the incidence of the disease in that population.

Cohort studies are characterized by their longitudinal design, where researchers collect data on participants based on their exposure to certain risk factors, their lifestyle choices, and other variables. The participants are then tracked to see how many ultimately develop cancer, which allows for the establishment of correlations between specific exposures and cancer outcomes. This study's large volunteer base and its focus on long-term health outcomes align perfectly with the definition and purpose of a cohort study.

In contrast, cancer cluster studies typically investigate unusual aggregation of cancer cases in a specific area or group but don’t track a wide population over time for a variety of risk factors. Case-control studies look backward after disease occurrence to identify and compare past exposures between cases and controls, while patterns of care studies analyze how different treatments are given across different patient populations rather than focusing on exposure and disease development tracking over time. Thus, the cohort nature of the Cancer Prevention II study highlights