Maths in a minute: Absolute versus relative risk

In 1995, thousands of women worldwide were scared by the news headlines that said taking contraceptives doubles the risk of blood clots. It’s reasonable to be worried about blood clots, but before taking action we should ask: double of what?

To understand the actual risk to us, we must look at what risk is being doubled in the first place. This is the absolute risk before and after whatever risk factor is being added. In this case the baseline absolute risk of developing blood clots in the next year for a woman not taking birth control was one in 7000 women. The experimental absolute risk for women on the pill was two in 7000.

Relative risk is the ratio of the two risks, in this case two, representing a doubling of the risk. That sounds dramatic, but the actual risks for an individual woman is still low – much lower than the risk of clots from pregnancy, for example.

Relative risks indicate how much larger or smaller one risk is relative to another. This can be useful at a population level, when making decisions about treatments, but does not indicate the size of the risk for individuals. For informed decisions, we need to know the absolute risks in all groups compared. You can read more about Individual risk versus population risk.

Hazard Risks 

While relative risk shows the difference in risk between two different groups, the hazard risk ratio compares the rate at which an event happens in two different groups. Understanding how fast the change occurs in two different groups can provide insight into how drastic the effect is. To find out more about hazard risk ratios, go to Understanding uncertainty: A 113% chance of death?.

Odds Ratio

The odds ratio compares the odds of an event happening in one group to the odds of it happening in another group.  Odds indicate the number of times the event occurs compared to the number of times it does not occur. For an in-depth explanation of how to calculate the odds ratio, go to Maths in a minute: Odds ratios.

Absolute risks, relative risks, hazard ratios, and odds ratios all help us understand scientific findings clearly and effectively. When equipped with this information people can make informed decisions in their lives.

About this article

Jasmine Fischbach is studying Economics and International Business Student at Knoxville, Tennessee in the United States. In spring 2026 she completed an internship with Sense about Science, an independent charity that promotes the public interest in sound science and evidence.