Absolute vs Relative Risk: The Numbers Behind Health Headlines

You see a headline: "New drug cuts heart attack risk by 50 percent!" It sounds amazing. But what does it mean for you? The answer depends on a number the headline left out. This guide explains the difference between absolute and relative risk, and how to make sense of health numbers.

Two ways to describe risk

Absolute risk

Absolute risk is the chance that something will happen to a person over a certain time. [1] For example:

• "2 in 100 people like you will have a heart attack in the next 10 years."
• "The risk is 2 percent over 10 years."

Absolute risk always needs a time period. A risk of 2 percent over 1 year is very different from 2 percent over a lifetime.

Relative risk

Relative risk compares the risk in one group with the risk in another group. [2] For example:

• "People who took the drug had half the risk of people who did not."
• "The drug lowered risk by 50 percent."

Relative risk does not tell you how big the risk was to begin with.

Why the difference matters

Here is a simple example, with made-up numbers to show the math.

• Without a drug, 2 in 100 people have a heart attack over 10 years.
• With the drug, 1 in 100 people have a heart attack over 10 years.

The relative risk reduction is 50 percent, because 1 is half of 2. The absolute risk reduction is 1 in 100, or 1 percentage point. Both are true. But "50 percent lower" sounds much bigger than "1 fewer person in 100".

Now imagine the starting risk is 40 in 100, and the drug cuts it to 20 in 100. The relative risk reduction is still 50 percent. But now the absolute reduction is 20 in 100. The same drug helps far more people in this group.

This is why your own starting risk is so important.

A real example

The Women's Health Initiative trial tested estrogen plus progestin in healthy women after menopause. Women taking the hormones had about a 26 percent higher rate of breast cancer than women taking a placebo. [5]

In absolute terms, this meant about 8 more cases of breast cancer for every 10,000 women each year: about 38 cases with hormones, compared with 30 with placebo. [5] Both numbers describe the same result. Each tells a different part of the story.

What doctors look at

Research shows that people, including doctors, often judge a treatment as more helpful when they see relative risk instead of absolute risk. [3] [4] For this reason, experts in risk communication suggest: [3] [4]

• Use absolute numbers, such as "1 in 100", alongside relative numbers.
• Use the same group size for comparisons, such as "out of 1,000 people".
• Always state the time period.
• Show both benefits and harms in the same format.

When your clinician talks about a treatment, they consider:

• Your starting risk, often from a risk calculator.
• The relative effect of the treatment from good studies.
• The absolute benefit for you, which is your starting risk times the relative effect.
• The absolute chance of side effects.

Other terms you may see

• Risk ratio or relative risk: risk in one group divided by risk in the other.
• Hazard ratio: a similar measure used in studies that follow people over time.
• Odds ratio: a related measure that is close to relative risk when the outcome is rare.
• Percentage points: the plain difference between two percentages. A drop from 2 percent to 1 percent is 1 percentage point.

Picture it with 100 people

Many people find it easier to picture risk with a group of people. Imagine 100 people like you.

• Without treatment, 2 of them have the problem over 10 years. 98 do not.
• With treatment, 1 of them has the problem. 99 do not.
• So 1 person out of 100 is helped. 98 would have been fine anyway. 1 has the problem despite treatment.

This picture also shows something else. Most people in the group did not need the treatment to stay well. That is not a reason to avoid treatment. It is a way to see the real size of the benefit, so you can weigh it against side effects, cost, and effort.

Some decision aids use pictures like this, with 100 small figures. Ask your clinician whether one exists for your decision.

How to read a health headline

• Ask "50 percent of what?"
• Look for the starting risk.
• Look for the time period.
• Look for the harms, in the same type of numbers.
• Check who was studied. Results may not apply to you.
• Be careful of single studies. One result can change when more studies are done.

Questions to ask your clinician

• What is my risk of this problem over the next 5 or 10 years?
• How much would this treatment lower my risk, in numbers out of 100?
• What are the chances of side effects, in the same kind of numbers?
• Is this based on people like me?
• How sure are we about these numbers?
• What happens if I wait or choose not to treat?

Bottom line

Absolute risk is your real chance of something happening. Relative risk compares two groups. A big relative change can mean a small real change when the starting risk is low. When you hear a health number, ask for the absolute risk, the time period, and the harms in the same terms.

Primary AI can show you the guideline and study sources behind this topic in plain language, so you can bring better questions to your appointment.