Research brief · 2026-09-09
Relative vs. Absolute Risk: The Most Common Trick in Health Headlines
A trial result can be reported two ways, and which one a headline picks changes how big the effect sounds. Relative risk reduction is a ratio, how much smaller one group's risk is than the other's, and ratios look large even when both risks are small. Absolute risk reduction is a subtraction: the actual difference in how many people were affected. Neither is wrong. They answer different questions.
The SELECT trial, published in the New England Journal of Medicine in 2023 by Lincoff and colleagues, is a clean example. Semaglutide is a prescription medication, and decisions about it belong with a physician. The trial randomized 17,604 people with existing cardiovascular disease and overweight or obesity to semaglutide or placebo. A primary cardiovascular event occurred in 6.5 percent on semaglutide and 8.0 percent on placebo, a hazard ratio of 0.80, summarized as a 20 percent relative reduction.
The absolute difference is 1.5 percentage points, and roughly 92 percent of the placebo group never had an event at all. One divided by 0.015 gives a number needed to treat of about 67: about 67 people treated for over three years to prevent one heart attack, stroke or cardiovascular death. In a high-risk group that is a real, meaningful effect. It is also a very different picture from twenty percent.
The framing works on harms too. Adverse events leading to discontinuation occurred in 16.6 percent on semaglutide versus 8.2 percent on placebo. Relatively, more than double. Absolutely, about eight extra people per hundred. Mixing an absolute benefit against a relative harm can make a trade-off look far better or worse than the numbers support, so compare like with like.
What the trial cannot tell you is the absolute benefit in a lower-risk group. Everyone in SELECT already had cardiovascular disease, and the 67-person figure is tied to this trial's follow-up. How much a one-in-67 chance matters to a person is a judgment for them and their physician. Sources are listed below.
Sources
- Lincoff AM et al., Semaglutide and cardiovascular outcomes in obesity without diabetes (SELECT trial, n=17,604)
- New England Journal of Medicine, 2023
- Peptide Proof, Relative vs. Absolute Risk: The Most Common Trick in Health Headlines (NNT worked from SELECT event counts)
- thepeptideproof.com, 2026
Peptide Proof reports what studies found. We do not give medical advice, dosing guidance, or treatment recommendations.