An observational study is research whereby variables are beyond the control of researchers. As the term suggests, an observational study observes the real world as opposed to creating a controlled experiment. They are often conducted when an experiment would be unethical. Observational studies often benefit from relatively low cost and large sample sizes. The following are common types of observational study.
A natural experiment is a real world situation that resembles an experiment. For example, a state that bans a pollutant where neighboring states do not.
Case-control StudyIdentifying groups on the basis of outcomes and then examining historical data for each group. For example, comparing the habits of adults who develop lung cancer with the habits of those who do not. Early studies that established the link between smoking and lung cancer used this method such as the landmark 1956 study "Lung cancer and other causes of death in relation to smoking" by Richard Doll and Bradford Hill.
Case StudyLooking at one example with a detailed analysis. For example, analysis of the life of Jeanne Calment a French woman who lived to the age of 122 years, considered the longest confirmed human lifespan. Case studies are based on interviews with her as she was quite famous by the age of 111 and was remarkably lively with intact cognitive abilities until the end of her life. She also participated in several medical examinations such as a CT scan at the age of 118.
Cross-sectional StudyCapture and analysis of data for a population at a point in time. For example, an analysis of the investment portfolios of small investors after a large scale market crash.
Longitudinal StudyObserving a population over a period of time. This can be a short or long timespan. For example, the Grant Study that looks at life outcomes for 268 Harvard college sophomores from the classes of 1939–1944 over a period of 75 years. Some longitudinal studies are not observational but are controlled experiments.
Prospective CohortA longitudinal study where a cohort, a group with a characteristic in common, are selected at the start of the study and then data collected over time as cross sections. For example, a study that looks at 50 year outcomes for people in a particular community who were breastfeed as babies versus those who were not. A prospective cohort begins with the cohort and collects future data.
Retrospective CohortA longitudinal study based on historical data for cohorts. For example, comparing the medical histories of people who have always lived in an area with poor air quality with those who have always lived in an area with relatively good air quality.
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