Relative Risks versus Changes in Absolute Risks

Absolute versus Relative Risks. In this video Professor Michael Spagat explains a nefarious technique for generating headline-grabbing health scares.

Here are some sample calculations that, hopefully, help you to understand relative and absolute risks better.

We can extract the following pieces of information from this nice little article.

1. 2.9 people out of 100 get bowel cancer by the age of 65 if they do not eat processed meat.
2. The bowel cancer rate rises to 3.4 out of 100 for people who eat 50 grams of red meat per day.

Thus, meat eaters face a relative bowel cancer risk, compared to non-meat-eaters, of 1.17. The calculations is simply 3.4/2.9 = 1.17. Note, however, that relative risks are often rendered in percentage terms which, in this case, comes out to 17%. This is calculated as:

((3.4 – 2.9) / 2.9) x 100 = 17%

It’s better, however, to express the risks in absolute terms. Facts 1) and 2) together imply that, roughly 1 person out of 200 meat eaters will get bowel cancer because of their meat consumption. Here’s the calculation:

(3.4 out of 100) – (2.9 out of 100) = 0.5 out of 100 which is the same as 1 out of 200.

So the change in absolute risk caused by meat consumption is 1 case of bowel cancer per 200 people.