Relative risk doesn't tell the whole story

Relative risk doesn't tell the whole story

Taking a single pill reduces the risk of heart attack by 34 per cent. Eating red meat increases the risk of breast cancer by 23 per cent. Getting more exercise reduces the risk of developing Alzheimer's by 30 per cent.

These are the sorts of headlines we see pretty much on a daily basis - and they can be quite scary. If there's something that is going to hugely increase our risk of developing cancer, dementia or any other kind of disease, we should all stop doing it immediately, right? On the other hand, if there's a pill, food or lifestyle change that can decrease our risk, we should all start doing it immediately, right?

But what do all these numbers and risks actually mean? Will making these changes really increase or decrease our risk of disease so dramatically?

When we see reports that something increases or decreases our risk of different diseases, it's usually reporting on the relative risk.

Relative risk, put more simply, means the likelihood of that event occurring in one group of people compared to another group of people who have different behaviours. For example, in smokers versus non-smokers, or in vegetarians versus meat eaters.

This relative risk, however, doesn't tell the whole story. To interpret these risks, we need to know something about the absolute risk of developing that disease in the first place. In simple terms, absolute risks tells us about the overall probability of an event taking place.

For example, the lifetime absolute risk of developing breast cancer is about 1 in 8 - or around12 per cent, or a risk of 0.12.

Let's take the example headline that eating red meat increases the relative risk of breast cancer by 23 per cent. Sounds pretty scary, and might make you think twice about that steak dinner. But to work out the absolute risk increase, it's 23 per cent of the absolute risk of 0.12. Which is around 0.15 (or 15 per cent). This is only a 3 per cent increase in absolute risk - which sounds a lot less scary than a 23 per cent increase in risk (and might make you feel OK about that steak after all)!

All these numbers and statistics can be really confusing - but understanding them can make you a savvier consumer.

There are certainly many things we can do, or stop doing, to live longer and healthier lives. But please don't be panicked - or tricked into spending your money on expensive lifestyle changes - by headlines reporting relative risks.

Dr Mary McMillan is a lecturer at the School of Science and Technology, University of New England