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An observational study is a type of research where the investigator observes subjects and measures variables of interest without assigning treatments to the subjects. The goal is to find associations between variables, such as lifestyle factors and health outcomes, without manipulating the study environment.

A retrospective study looks backward in time, usually using medical records and interviews with patients who already have a known outcome. Researchers identify subjects with and without a particular condition and then look back to see if there are differences in exposure to a suspected risk factor.

Cross-sectional studies analyze data from a population at a specific point in time, while prospective studies follow subjects over time to observe future outcomes. Understanding these differences helps in identifying the study design, which affects the interpretation of results and potential biases.