A retrospective cohort study is a type of observational study that looks back in time to examine the relationship between an exposure (e.g. a risk factor, treatment, or disease) and an outcome (e.g. a disease or health condition) in a group of individuals. In a retrospective cohort study, the researcher identifies a group of individuals who have been exposed to a particular factor (the "exposed group") and a group of individuals who have not been exposed (the "unexposed group"), and then compares the outcomes between the two groups.
A retrospective cohort study is different from a prospective cohort study in that the exposure and outcome have already occurred in the past. This means that the researcher cannot control the exposure and must rely on existing data and records to identify the exposed and unexposed groups. The researcher can compare the groups to identify any differences in outcomes, and use statistical methods to estimate the relative risk of the outcome associated with the exposure.
Retrospective cohort studies can be useful for studying rare diseases or exposures, or when
a long latency period exists between the exposure and outcome, but they may be limited by the availability and quality of data, as well as potential biases.
Examples of retrospective cohort study:
∙Investigating the relationship between smoking and lung cancer by comparing the medical records of a group of patients with lung cancer to a group of patients without lung cancer.
∙Examining the effects of a specific medication on the risk of heart attack by comparing the medical records of patients who were prescribed the medication to a group of similar patients who were not prescribed the medication.
回顾性队列研究是一种观察研究，它回顾过去，检查暴露（如危险因素、或疾病）和结果（如疾病或健康状况）之间在一个体中的关系。在回顾性队列研究中,研究者识别了一组已经暴露于特定因素的个体(“暴露组”)和一组未暴露的个体(“未暴露组”),然后比较两组结果之间的差异。
回顾性队列研究与前瞻性队列研究不同，因为暴露和结果已经在过去发生。这意味着研究者
不能控制暴露，必须依靠现有的数据和记录来识别暴露组和未暴露组。研究者可以将两组进行比较，以确定结果之间的差异，并使用统计方法来估算与暴露相关的结果的相对风险。
回顾性队列研究对于研究罕见疾病或暴露是有用的，或者当暴露和结果之间存在较长的潜伏期时，但它们可能受到数据的可用性和质量以及潜在偏差的限制。
回顾性队列研究的例子:
∙xposed通过比较肺癌病人和没有肺癌病人的医疗记录来研究吸烟与肺癌之间的关系。
∙通过比较被处方药物的病人和未处方药物的病人的医疗记录来检查特定药物对心脏病风险的影响。

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