Absolute vs Relative Effects: Reading Percentages in Health News
A 25% lower risk can mean 1 percentage point or far more. Learn the difference between relative and absolute effects, how number needed to treat works, and what to ask when a headline gives only a percentage.
Content last updated . This page has not been medically reviewed.
The same result, told two ways
Health news often reports a result as a percentage change in risk. That number alone can mislead, because it hides how big the risk was to begin with. The Cochrane Handbook gives a clear example: an intervention that reduces the relative risk of death by 25% and one that reduces the risk of death by 1 percentage point can describe the same benefit, a drop in risk from 4% to 3%. [1] This page explains the terms and the questions to ask. It is education, not medical advice.The terms
| Term | What it means | Example from the 4% to 3% case |
|---|---|---|
| Relative risk reduction (RRR) | The percentage by which risk falls compared with the comparison group. A risk ratio of 0.75 equals an RRR of 25%. [1] | 25% |
| Absolute risk reduction (ARR) | The difference in risk between groups, shown as a percentage, a decimal or a count such as 10 out of 1000. [1] | 1 percentage point (10 per 1000) |
| Number needed to treat (NNT) | The expected number of people who need to receive the intervention instead of the comparator for one additional person to avoid (or have) an event over a stated time. [1] | About 100 (derived: 1 divided by 0.01) |
The NNT is a derived figure. It is a comparison against a specific comparator, not a general property of one intervention, and it is an expected value: an NNT of 10 does not mean one additional event in every group of 10. [1]
Why headlines prefer the relative number
A Cochrane systematic review of 35 studies on how risk is presented compared the formats. Relative risk reduction was perceived as larger than absolute risk reduction and was more persuasive, yet it was not better understood. Compared with NNT, relative risk reduction was better understood but also perceived as larger and more persuasive. [2] In plain terms, relative figures sound bigger, whether or not the benefit is big. The same review found that people understood natural frequencies (for example, "10 out of 1,000") better than probabilities. [2]A checklist for any percentage in a health story
- Percent of what? Ask for the starting risk, ideally as a count out of 1,000 or 10,000.
- Compared with what? A placebo, usual care, or nothing?
- Over what time? Risks depend on the period measured. [1]
- Which outcome? Prefer outcomes you would notice. See surrogate vs patient-important outcomes.
- What about harms? Relative and absolute numbers also apply to side effects.
- Who was studied? Baseline risk differs by group, so results may not apply to you. See efficacy vs real-world effectiveness.
- Is the difference statistically and clinically meaningful? See statistical vs clinical significance.
Percentages also appear in marketing. See research hype and press releases and how to read health product marketing claims. Questions about your own risk are best discussed with a clinician.
Frequently asked questions
What is the difference between absolute and relative risk reduction?
Relative risk reduction is the percentage cut compared with the comparison group; absolute risk reduction is the plain difference in risk, such as 1 percentage point. [1]
Why can a big percentage mean a small benefit?
Because the percentage applies to the starting risk. A 25% relative reduction can mean going from 4% to 3%. [1]
What does number needed to treat mean?
It is the expected number of people who need the intervention rather than the comparator for one additional person to avoid or have an event in a given time. [1]
Do people understand relative or absolute numbers better?
A Cochrane review found relative risk reduction was perceived as larger and more persuasive than absolute risk reduction, with little or no difference in understanding. [2]
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Why the starting risk matters
A made-up illustration, not a real study: a risk of 20 in 1,000 cut by 25% becomes 15 in 1,000, a drop of 5 per 1,000. A risk of 2 in 1,000 cut by 25% becomes 1.5 in 1,000, a drop of half a person per 1,000. The headline percentage is identical.