Health Services Research (HSR) Methods
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Historical Controls

A group of patients chosen from a sample who were observed in the past. In this type of study, patients received new treatment form the experimental group and patients who received standard treatment serve as "controls."

Selection bias might be a concern in this approach because subjects receiving new treatment might not be comparable to subjects that received traditional treatment.

Further Reading
Gordis, L. 2000. Epidemiology, 2nd ed., Philadelphia, PA: Elsevier Science.