Final answer:
The study that selects subjects based on the presence or absence of a condition of interest is a case-control study. Researchers compare past behaviors of cases and controls to identify associations with the condition.
Step-by-step explanation:
When subjects are selected for a study based on whether or not they have the condition of interest, the type of observational study that is typically being referenced is a case-control study. This is so because, in a case-control study, groups of subjects are selected retrospectively: those who have the trait of interest (cases) and those who do not have the trait (controls). Researchers study these groups by examining their past behaviors and trying to find correlations between those behaviors and the current status of having or not having the condition.
In contrast, a cohort study involves selecting a group of similar subjects at the start of the study, which is then followed over time. This is a prospective study, meaning that data are collected periodically into the future, allowing for more accurate data collection and a better understanding of the progression of disease states or conditions.
On the other hand, a cross-sectional study captures information about subjects at one point in time without following up. An experimental study involves the manipulation of independent variables and the assignment of subjects to different conditions, such as in a randomized controlled trial.
Case-control studies are useful in establishing associations between potential risk factors and diseases or conditions, particularly when conducting a prospective study is impractical or unethical. However, the accuracy of the data may be affected by subjects' recall bias and other limitations inherent in retrospective analysis.