http://www.redflagsweekly.com/kendrick/2003_jul10.html

MALCOLM KENDRICK, MD

July 10, 2003

HOW RISKY IS A RISK?

By Malcolm Kendrick MbChB, MRCGP (email - malcolm@llp.org.uk ) 

I have only just recovered from the idea that everyone in the whole world over the age of fifty-five should spend the rest of their lives on six different medications, all stuck together in one great big pill. You may have seen the non-story about the non-existent polypill peddled in the British Medical Journal (BMJ). I was stimulated to look again at the concept of risk.

The authors of the madcap polypill article in the BMJ made the claim that taking their polypill would reduce the risk of dying of coronary heart disease (CHD) by 80%. Whether or not you believe their figures — and I don’t — I sense that this figure of 80% would be taken by most people to mean that eighty out of one hundred people would be saved from death.

Yet, that is not what it means at all, for this figure is a relative risk reduction figure. And a relative risk reduction means nothing, by itself. However, because everyone’s eyes glaze over whenever you start talking about statistics, most researchers manage to get away with using relative risk reduction figures when, in reality, they should be shot for doing so.

Now, here’s a challenge. The challenge to make an article about statistics interesting….. Okay, that’s not possible. But maybe a little bit interesting?

When you talk about a risk, you need to know the absolute risk of a thing happening. For example, the risk of getting struck by lightening. I don’t actually know what that risk is, but I would imagine it is about one in five million. But again, that figure means nothing unless you put a time scale on it. Is this a one in five million risk over a hundred years, or one year?

If you don’t put a time scale in, you can claim anything you like. For example ‘The Earth will be hit by a big Asteroid. This is one hundred percent certain — shock claim from Astronomer.’ Read all about it. And of course, this is true. The Earth will be hit by a big Asteroid, sometime in the next three billion years or so. The odds ratio for this event is 1 = 100% certain. I am even willing to take a bet on it.

So, I must define risk in two ways, the possibility of the thing happening, and the time period during which that thing will happen. With lightening strikes, this is about a one in five million risk, over a five year period. Not high.

However, people don’t tend to bend statistics by ignoring the time factor that often. Unless they want money to fund their Asteroid defence system. A snip at five trillion dollars, plus VAT. What generally happens is that people inflate the risk in the following way. For example, the chances of dying of lung cancer, for a non-smoker, are about 0.1%, lifetime risk. If, however, you live with a heavy smoker, your chances will increase to about 0.15%. (These figures are for illustration only).

Now you can report this in two ways. You can state that passive smoking can increase the risk of lung cancer by 0.05% - one in two thousand. Or, you can state that passive smoking increases the risk of lung cancer by fifty per cent. Both figures are correct. If you are an anti-smoking zealot, then I would imagine you would prefer to highlight the second figure. The relative risk figure.

And when it comes to reducing cardiovascular risk, exactly the same procedure is used (in reverse). Let’s say that the chance of dying of CHD over five years, in a healthy fifty-five year-old, is 1%. By reducing this risk to 0.2%, you will have reduced the relative risk of dying of CHD by 80%. In this way a 0.8% absolute risk reduction is hyped up as an 80% reduced risk of dying of CHD. Mangling statistics is easy when you know how. It’s even fun.

Anyway, now you know the difference between a relative risk and an absolute risk, and I hope this makes it easier for you to hack your way through the misinformation that spews forth from the great medical research machine.