Healthy User Effect

In the world of health-related research, there’s growing evidence that many studies have shown exaggerated results not because the pharmaceutical or other treatment is just really, really good, but because the test subjects who actually stuck with the intervention being tested (the drug, the routine, the lifestyle change, etc) were already themselves biased toward healthy behaviors.

In practice, that means if you have people take a specific vitamin supplement to see if it helps prevent the common cold, your data might show that the people who took the supplement as advised, in the correct dosage and at the correct cadence, will tend to report better outcomes than those who did not do things correctly (or those in the control group).

This might (understandably) then be presented as evidence that taking said vitamin in the advised dosage helps prevent the common cold. But what may have happened, instead, is that the people who adhered to the advised regimen are actually just healthier, overall, and more disciplined in how they approach health-related things. They’re not just more likely to take pills correctly, they’re also more likely to work out regularly, eat decently healthy meals, avoid tobacco, wear their seatbelts when driving, and so on.