What is the primary characteristic of a case control study?

The primary characteristic of a case control study is the comparison of individuals who have a specific condition or disease (the cases) with individuals who do not have the condition (the controls). This type of observational study is designed to identify and compare the presence of potential risk factors or exposures between the two groups to determine if there is an association between the exposure and the disease.

In a case control study, researchers typically select participants based on their disease status rather than their exposure status, which allows for the efficient study of rare diseases. This design helps to explore the relationship between specific exposures and the likelihood of developing the disease, making it particularly valuable in understanding epidemiology and potential causal factors for health conditions.

The other options describe features typical of different study designs. Following participants over time is characteristic of prospective cohort studies. Random assignment is a feature of randomized controlled trials, which aim to minimize bias in treatment comparisons. Computing incidence rates directly is more associated with cohort studies where participants are followed over time to measure new cases. Thus, the comparison of cases with controls is the defining element of case control studies.