Relative risk and absolute risk

This is the single most useful thing to understand about health headlines, and it explains more misleading claims than dishonesty ever has. Almost nobody in the chain is lying. They are choosing the larger of two true numbers.

The two ways to describe one result

Suppose a study finds that in the group taking something, 1 person in 1,000 developed a condition, and in the group not taking it, 2 people in 1,000 did.

Relative risk reduction: 50 percent. The rate halved. This is completely accurate.

Absolute risk reduction: 0.1 percentage points. One fewer person per thousand. Also completely accurate.

The first number will be in the headline. The second number is the one you would need to decide whether to take it.

Why relative risk dominates

Not conspiracy. Incentives, all pointing the same way. Researchers report relative reduction because it is the conventional way to express an effect and it makes a finding legible across studies of different sizes. Press offices use it because it is the more striking number. Journalists inherit it from the press release. Supplement marketers use it because it is the largest true number available.

By the time it reaches a social feed, four people have passed along an accurate figure and nobody has stated the base rate it applies to. That omission does all the work.

Where it matters most in longevity

Longevity claims are unusually vulnerable to this, because the conditions being reduced are often rare in the population being studied and the timescales are long. A rare outcome has a tiny base rate, so almost any effect on it produces a dramatic relative number.

A 40 percent reduction in an outcome that affects 3 people in 10,000 is a real finding and a poor reason to change your week. The same 40 percent applied to something affecting 1 person in 5 would be genuinely important. Nothing in the phrase "40 percent reduction" distinguishes them.

The reverse trick

The same asymmetry runs the other way when the news is unfavourable. Risks a seller does not want emphasised get reported in absolute terms, where they sound negligible, and benefits get reported in relative terms, where they sound decisive. Seeing both framings in the same piece of marketing, pointed in opposite directions, is a reliable signal that somebody chose deliberately.

How Longevity Claim Verification handles it

Every effect size we report is given in absolute terms alongside whatever relative figure the source used, and where the underlying paper does not provide enough information to calculate the absolute figure, that is stated as a limitation rather than skipped. A claim we cannot express in absolute terms is a claim nobody can act on responsibly, including us.