Absolute and relative risk

About the “What researchers mean by...” series

This research term explanation first appeared in a regular column called “What researchers mean by…” that ran in the Institute for Work & Health’s newsletter At Work for over 10 years (2005-2017). The column covered over 35 common research terms used in the health and social sciences. The complete collection of defined terms is available online or in a guide that can be downloaded from the website.

Published: November 2006

The media often mentions risk when reporting on research, but this can sometimes be misleading. For example, if a newspaper article reports on research that shows a certain gene puts people at 800 per cent increased risk of getting a blood clot, and you have that gene, you would likely be very worried reading this news. But should you be?

Understanding how risk is expressed can help determine a study's significance, or a person's chance of illness, injury or recovery. Risk can be explained in terms of absolute or relative risk. Here's a look at the difference between these terms.

Absolute risk

Let's say a study of 100 workers in factory A revealed that 20 workers experienced back pain on the job. In factory B, 30 workers in a similar workplace of 150 workers developed back pain. The absolute risk of developing back pain is simply the percentage of people affected. This is 20 per cent in both groups. In scientific terms, absolute risk is the number of people experiencing an event in relation to the population at risk.

Relative risk

Relative risk is a comparison between two groups of people, or in the same group of people over time. It can be expressed as a ratio. In the example above, the relative risk of developing back pain — comparing factory A and factory B — is 20:20 or one. That is, workers in factory A are no more (or less) likely to have back pain than workers in factory B. It's 20 per cent for both groups.

Now suppose workers in factory A were to receive exercise therapy for half an hour each day. One year later, we find that only eight of 100 workers have back pain, while the rate in factory B remains the same at 20 per cent.

The ratio now changes to 8:20. Eight is the risk per 100 workers in factory A. Twenty is the risk per 100 workers in factory B. If we divide eight by 20, this gives us 0.40, or 40 per cent. In other words, the relative risk of developing back pain in factory A is now 40 per cent of the risk in factory B.

Risk reduction

How much did the risk of back pain change due to the exercise therapy intervention? Again, this can be calculated two ways, using absolute and relative risk reduction.

Absolute risk reduction is the difference in the percentage of people who are affected. Again, recall that before the intervention, 20 per cent of workers in factory A developed back pain. Afterwards, eight per cent did. The difference is 12. Therefore, the intervention resulted in an absolute risk reduction of 12 per cent.

The relative risk reduction is the change in relative risk. Recall that before the intervention, the relative risk was one for both factory A and B. After the intervention, it dropped to 0.40. The difference is 0.60. In other words, the intervention resulted in a 60 per cent reduction in relative risk.

Which is better?

Risk expressed either way is correct. In our example, the relative risk reduction of 60 per cent appears larger than the absolute risk reduction of 12 per cent. It often helps to look at both types of risk to see how significant a change is.

For example, say the absolute risk of a work injury is two per 100 workers. Due to an intervention, it drops to one injury per 100 workers. This yields a relative risk reduction of 50 per cent. Overall, in absolute terms, this means one less injured worker per 100.

In another case, say the absolute risk of injury is 50 per 100 workers, but drops to 25 injuries per 100 workers. This will also result in relative risk reduction of 50 per cent. However, this translates to 25 fewer injured workers per 100. Even though the relative risk reduction is the same in both cases, the second intervention has a greater impact overall.

Let’s go back to the example of the newspaper article on the risk of clotting due to the presence of a certain gene. The article reported on the relative risk; i.e. you are 800 per cent more likely to get a blood clot relative to those who don’t have the gene. However, knowing the absolute risk is important. If the absolute risk of getting a blood clot is one in 1,000, and you are at an 800 per cent increased risk of getting a blood clot because of the presence of the gene, your risk is now eight in 1,000. So there’s still a very very good chance you won’t get a blood clot at all.

Source: At Work, Issue 46, Fall 2006: Institute for Work & Health, Toronto [Revised in 2017]