Medical tests and treatments
How to interpret benefits and harms: absolute vs relative risk, NNT, and time windows.
Medical risks: tests and treatments
Medical interventions have benefits and harms. Good decisions compare both—using the same unit and time window.
How clinicians communicate risk
- Common vs rare side effects
- Short-term vs long-term risks
- Population-average vs personalized risk
Absolute risk reduction (ARR)
Example: If 10 out of 100 get an event without treatment and 6 out of 100 with treatment, ARR is 4 out of 100 (4%).
Relative risk reduction (RRR)
Using the same example: going from 10 to 6 is a 40% reduction. That sounds big, so always ask for ARR too.
Number needed to treat (NNT)
NNT is “how many people need the treatment for 1 person to benefit.” In the example above, ARR is 4%, so NNT is about 25 (because 1 / 0.04 = 25).
Ask your clinician: “Out of 100 people like me, how many benefit? How many are harmed? Over what time period?”
See Personal case study for a 72-year timeline showing trade-offs, diagnostic delay, and cumulative risk.