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Prospective Study
The term prospective study refers to a study design in which the documentation of the presence or absence of an exposure of interest is documented at a time period preceding the onset of the condition being studied. In epidemiology, such designs are often called cohort studies. Characteristic features of these designs include initial selection of study subjects at risk for a condition of interest, but free of disease at the outset. The occurrence/ nonoccurrence of the condition is then assessed over a time period following recruitment. The time sequencing of the documentation of the exposure and outcome is an important feature and is related to a commonly cited form of evidence toward drawing causal inference about the relationship between the exposure and the outcome. The terms prospective study and cohort study also are usually used for observational or nonexperimental designs in which the investigator has not assigned study participants to the levels of exposure or intervention, distinguishing this design from prospectively conducted experiments or trials. In this entry, the distinction between prospective studies and retrospective studies is provided, followed by a discussion of the advantages and analysis of prospective studies and examples of influential studies.
Prospective versus Retrospective Studies
Prospective studies may be contrasted against retrospective studies, a term that is sometimes applied to case-control studies, in which subjects are selected based on the presence or absence of disease, and exposure history is then documented retrospectively. Prospective studies, however, may use data collected entirely or partially in the past (such as through analysis of existing records for exposure or both the exposure and outcome); in this case, the distinguishing feature remains that the exposure of interest existed, and was documented, prior to the onset of the condition. Across disciplines, there is some overlap in the use of the terms prospective study, cohort study, and panel study. All of these terms imply a prospective design (appropriate time sequence in the assessment of exposure and outcome).
In early publications, many authors used the term prospective study synonymously with the term cohort study. In essence, retrospective and prospective can be terms that are used in conjunction with study designs such as case-control and cohort. It is possible to have both prospective and retrospective case-control studies. A case-control study would be considered prospective if the exposure measurements were taken before the outcome is ascertained (often called a nested case-control study). Likewise, it is possible to have both a prospective and retrospective cohort study. A cohort study can be classified as retrospective if the outcome has occurred before the exposure is ascertained. Cohort studies are often used as the foundation for related study designs such as case-control studies nested within cohort studies and case-cohort designs.
Confusion can exist given that both case-control and cohort designs can have both prospective and retrospective measurement elements. The prospective and retrospective distinction is often applied to the timing of subject identification and does not necessarily refer to the timing of outcome assessment relative to exposure assessment. For example, sometimes a cohort study is identified as a retrospective cohort study, indicating that it is a cohort study that involves the identification and follow-up of subjects, but subjects are identified after completing the follow-up period.
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