In a cohort study, the sample consists of subjects (people) who all possess a given characteristic or set of characteristics. This virtually guarantees that a significant number of subjects will develop the condition being studied. Additionally, the sample size may be small, compared to traditional research methods, while maintaining significance in the scientific research community. A retrospective cohort study may also follow a cohort (group of people with a common characteristic) who are receiving treatment for their condition, and then judge the treatment efficacy.
A retrospective cohort study attempts to prove or deny the risk of something, such as an illness or disorder, by observing a group of people with common characteristics. Ultimately, there is a premise that a certain characteristic creates vulnerability for the condition, such as exposure to a drug while in the womb causing the disorder or illness to develop later. A retrospective study implies that information is obtained, and then judgments are made after the fact. Some retrospective cohort studies observe subjects for a long time while attempting to find correlations with multiple conditions. Other studies target specific cause and effect scenarios.
In retrospective cohort studies, multiple possible causes may be evaluated if they occurred under the same conditions as the primary focus, although only one primary condition is generally the focus of the study. The researchers also have a greater opportunity to standardize the study conditions and manipulate or observe the outcome, which reduces the benefit of randomization and can increase bias. However, this is generally favorable for this type of study. Retrospective cohort studies are less prone to bias than standard cohort studies.
Cohort studies remove the need for random sampling in the given research, which makes finding a research cohort and control group significantly easier. Generally speaking, a retrospective cohort study is quick, although it can be a longitudinal study and take more time. The cost is relatively inexpensive when compared to traditional research methods. With one primary variable and the ability to look back and review the outcomes rather than forming and testing hypotheses, retrospective cohort studies are also considered easier to conduct.
Retrospective cohort studies may call for a large sample size, depending on the condition and circumstances studied. This can increase the costs, making them comparable, if not more expensive than, other research styles. The quickness of some cohort studies may be a disadvantage if the condition or outcome requires more time to develop. In addition, confounded variables may become a problem when selecting the cohort and control group subjects. If everyone who starts a study does not finish it, the results can be affected--especially for longitudinal studies.
- "Practical Research: Planning and Design"; Paul D. Leedy, Jeanne Ellis Ormrod; 2010
- Social Research Methods: Cohort Studies
- University of Pennsylvania -- ITMAT: Study Designs Used in Applications of Automated Databases for Epidemiologic Research
- The George Washington University Medical Center Health Sciences Library: Study Design 101
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