This kind of bias may be at work in studies that find an unexpected benefit associated with treatment. It refers to the fact that people who are health conscious—those who are more likely to get regular check-ups, to comply with their doctors’ orders, to take prescriptions as directed, etc.—usually fare better health-wise than those who do not or cannot.
Deeper dive
For example, years of observational studies concluded that seniors who got flu shots had half the risk of dying from any cause during the subsequent flu seasons compared to those who didn’t get flu vaccines. Thus, researchers reasoned, flu shots appeared to be a powerful way to slash an elderly person’s death risk. But studies that dug a little deeper, examining individual medical records for other signs of health and frailty, found that seniors who got flu shots were simply healthier to begin with than those who didn’t get their annual vaccines. It was healthy users who were surviving the winter months. Those studies found flu shots had little effect on overall survival.