In which type of study do researchers directly intervene to observe outcomes?

In a randomized controlled trial (RCT), researchers actively intervene by assigning participants to different treatment groups to compare outcomes. This allows for a controlled environment where the effects of specific interventions can be measured more accurately than in observational studies. The randomization process helps to minimize bias, ensuring that differences in outcomes can be more confidently attributed to the interventions being tested rather than other confounding variables.

In contrast, case control studies, cohort studies, and longitudinal studies do not involve direct intervention by researchers. Case control studies look retrospectively at individuals with and without a specific outcome to explore potential exposures. Cohort studies follow groups over time to see how different exposures affect outcomes, while longitudinal studies track the same individuals over an extended period. None of these approaches involve the researchers manipulating the variables as seen in randomized controlled trials.