What are the negatives of historical controls vs. contemporaneous ones?
The key idea is to compare people in the same time frame. For example, in a comparison of surgery versus chemotherapy for breast cancer, you wouldn’t want to use surgery patients from 20 years ago as a control group to compare against a current chemo group. The effectiveness of surgery has increased dramatically over 20 years, women’s health and awareness of breast cancer has increased dramatically over 20 years, and the practices of the medical profession have changed dramatically over 20 years. The goal is to eliminate as many confounding factors as possible, and using people in the same time frame is one way to eliminate time as a confunder.
