![]() ![]() In retrospective studies, the outcome of interest has already occurred (or not occurred – e.g., in controls) in each individual by the time s/he is enrolled, and the data are collected either from records or by asking participants to recall exposures. The terms “prospective” and “retrospective” refer to the timing of the research in relation to the development of the outcome. Prospective versus retrospective study designs As described earlier, analytical studies can be observational (if the exposure is naturally determined) or interventional (if the researcher actively administers the intervention). In these studies, the researcher assesses the effect of an exposure (or intervention) on an outcome. Examples of descriptive studies include a survey of dietary habits among pregnant women or a case series of patients with an unusual reaction to a drug.Īnalytical studies attempt to test a hypothesis and establish causal relationships between variables. Examples of descriptive studies include case reports, case series, and cross-sectional surveys (please note that cross-sectional surveys may be analytical studies as well – this will be discussed in the next article in this series). These do not try to answer questions or establish relationships between variables. For example, a study could randomly assign persons to receive aspirin or placebo for a specific duration and assess the effect on the risk of developing cerebrovascular events.ĭescriptive (or nonanalytical) studies, as the name suggests, merely try to describe the data on one or more characteristics of a group of individuals. This intervention could take many forms – for example, administration of a drug or vaccine, performance of a diagnostic or therapeutic procedure, and introduction of an educational tool. ![]() Interventional studies are experiments where the researcher actively performs an intervention in some or all members of a group of participants. Observational studies can be either descriptive (nonanalytical) or analytical (inferential) – this is discussed later in this article. Any variable that occurs simultaneously with or following the outcome cannot be causative, and hence is not considered as an “exposure.” In these studies, the investigator did not play any role in determining the smoking or dietary habit in individuals.įor an exposure to determine the outcome, it must precede the latter. For example, looking at the incidence of lung cancer in smokers versus nonsmokers, or comparing the antenatal dietary habits of mothers with normal and low-birth babies. The researcher does not do any active intervention in any individual, and the exposure has already been decided naturally or by some other factor. Observational studies are those where the researcher is documenting a naturally occurring relationship between the exposure and the outcome that he/she is studying. Observational versus interventional (or experimental) studies If a drug had been started in some individuals but not in the others, before the study started, this counts as exposure, and not as intervention – since the drug was not started specifically for the study. Typically, the term “exposure” is used when the “causative” variable is naturally determined (as in observational studies – examples include age, sex, smoking, and educational status), and the term “intervention” is preferred where the researcher assigns some or all participants to receive a particular treatment for the purpose of the study (experimental studies – e.g., administration of a drug). The outcome (or predicted or dependent) variable develops as a consequence of the exposure (or intervention). It is also referred to as the independent or the predictor variable. Exposure (or intervention) refers to the risk factor whose effect is being studied. Here, the question is whether one variable is associated with or responsible for change in the value of the other variable. Exposure (or intervention) and outcome variablesĪ large proportion of research studies assess the relationship between two variables. ![]()
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