Diet & Weight Magazine

Understanding Absolute and Relative Risk

By Dietdoctor @DietDoctor1

Although it seems as if numbers should be objective and trustworthy, there are many ways that they can be used to distort the truth. Entire books have been written about this subject... like the 1954 classic, How to lie with statistics.

With chronic diseases, one of the most deceptive "sleights of hand" with numbers is the use of relative risk to report the size of an effect found in a study. Relative risk confuses unsuspecting readers; it can make even a tiny effect seem very large.

Let's take a look at the differences between absolute risk and relative risk.

Absolute risk is the chance, or probability, that a specific health event will happen to you (or to anyone in a similar group of people).

Relative risk, on the other hand, is the chance, or probability, that people in one group will experience a health event compared to people in another group.

You might ask, "Why does this matter?" Well, the two numbers - in exactly the same treatment scenario - can appear drastically different, with absolute risk being a very small number and relative risk seeming to be very large. This can distort perceptions about how successful and worthwhile a specific medical treatment may be for you personally.

For example, say in a group of 100 people, there is a risk that two people will have a heart attack. If you are in that group, your absolute risk of a heart attack is 2%. Now, let's say researchers conduct a study of a drug treatment for this condition. One group of 100 people gets a placebo (the control group), and another group of 100 people gets the drug - which could have significant side effects.

The study finds that while the control group had the expected two heart attacks, the treatment group had just one. Also, in the treatment group, 15 people experienced significant side effects. The absolute risk decreased by only 1% but the relative risk decreased by 50%! Your doctor might tell you "people who took this drug had a 50% better outcome." But your risk is really reduced by just 1% if you take the drug and there will be a 15% risk of experiencing the side effects.

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