- Starting risk
- 2 in 100
- Comparison risk
- 1 in 100
- Absolute difference
- 1 percentage point
- Relative reduction
- 50%
1 percentage point absolute difference; a 50% relative reduction.
Hypothetical risks over the same period. These numbers do not describe a clinical trial.
Start with the denominator
Imagine a hypothetical study: 2 of every 100 people in one group experience an event, compared with 1 of every 100 in another group, over the same period. These are invented numbers for explaining the arithmetic.
Two correct numbers, two different questions
The absolute difference is 2% minus 1%: one percentage point. The relative reduction is that difference divided by the starting risk: 1% ÷ 2% = 50%. Both describe the same example. The relative number alone does not tell you how common the event was.
Carry the time frame with the number
Ask which event was measured, over how long, and in whom. A one-year risk and a ten-year risk cannot be substituted for each other. A study average is also not a prediction for a particular person.
What to look for in the source
Find the event counts, group sizes, follow-up period and uncertainty interval. A hazard ratio describes events over time; it should not automatically be read as a simple difference between percentages. For a real study, use its reported statistical analysis rather than applying this toy example mechanically.
Limits of this interpretation
- The worked example is invented for education.
- It does not model censoring, varying follow-up or individual risk.
Sources & transparency
This is an original educational explainer. Its worked examples are illustrative and do not describe a clinical trial.
Prepared with AI assistance. Worked examples are illustrative, not estimates of personal medical risk.
Clinical review: Not applicable to this educational guide
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