Retrospective cohort studies, defined as a follow-up study in which outcome occurred prior to study onset, are common in craniofacial outcomes research and will continue to be prevalent given the increasing availability of secondary datasets and inherent prospective study limitations. However, if available data are not adequately measured, or necessary variables are absent, retrospective cohort studies can be particularly prone to bias. This brief communication aims to highlight the primary sources of bias, including measurement error, selection bias, and confounding. Each source is clearly defined, examples pertinent to craniofacial outcomes are provided, and mitigation strategies are discussed.
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