Absolute and relative risk

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.

Continue reading the article on Institute for Work & Health.

Video taken from TEDEd