Disadvantages
Retrospective studies have disadvantages vis-a-vis prospective studies. Among the disadvantages are that some key statistics cannot be measured, and significant biases may affect the selection of controls. Additionally, major biases with retrospective cohort studies can impact the recall of former exposure to risk variables. Among the biases which can negatively impact the veracity of this type of study are selection bias and misclassification or information bias as a result of the retrospective aspect. With retrospective studies, the temporal relationship is frequently difficult to assess. Further, those conducting retrospective studies can't control exposure or outcome assessment, but instead need to rely on others for accurate record-keeping. This is particularly problematic because it can be very difficult to make accurate comparisons between the exposed and non-exposed. Retrospective studies also can need very large sample sizes for rare outcomes.
Read more about this topic: Retrospective Cohort Study