Overview
This lecture discussed how health interventions are often misunderstood due to confusing relative risk reduction with absolute risk reduction, and introduced the concept of number needed to treat (NNT).
Understanding Risk Reduction
- Health interventions are commonly discussed in binary terms (works/doesn't), but real effectiveness varies.
- Relative risk reduction measures the percentage decrease in risk, comparing new risk to old risk.
- Absolute risk reduction is the difference between old and new risk, providing a clearer picture of benefit.
Examples of Relative vs Absolute Risk
- Reducing heart attack risk from 50% to 25% is a 25% absolute risk reduction and 50% relative risk reduction.
- Reducing brain cancer risk from 0.5% to 0.25% is a 0.25% absolute risk reduction, still 50% relative risk reduction.
- Relative risk reductions can make benefits seem larger than they are, especially in studies and news.
Absolute Risk Reduction in Research
- Mediterranean diet study: 1.7% absolute risk reduction in major cardiovascular events.
- Lung cancer screening (high-risk smokers): 0.5% absolute risk reduction.
- Aspirin for primary prevention: only 0.06% absolute risk reduction.
Number Needed to Treat (NNT)
- NNT shows how many people must receive an intervention for one person to benefit (calculated as 100 / absolute risk reduction).
- Heart attack drug: NNT = 4 (treat 4, one benefits).
- Brain cancer drug: NNT = 400 (treat 400, one benefits).
- Mediterranean diet: NNT = 61 over 5 years for one to benefit.
- Lung cancer screening: NNT = 217.
- Aspirin: NNT = 1,667.
Implications for Health Decisions
- Most individuals do not benefit from interventions; the odds of personal benefit can be very low.
- High NNTs mean many people receive no benefit, raising questions about cost and side effects.
- People rarely understand their actual chance of benefit when making health decisions.
Key Terms & Definitions
- Relative Risk Reduction — The proportional decrease in risk due to an intervention.
- Absolute Risk Reduction — The direct difference in risk before and after an intervention.
- Number Needed to Treat (NNT) — The number of people who must receive a treatment for one person to benefit.
Action Items / Next Steps
- Consider how absolute risk reduction and NNT apply to health advice you receive.
- Await next week’s lecture on weighing benefits vs. potential harms of interventions.