Relative Risk Vs. Absolute Risk
by Nancy Ryan
Understanding how research data are presented is essential to
understanding its significance. One source of confusion is the
difference between relative risk and absolute risk. If a study
reports that a drug “reduces your risk of developing breast cancer by
50%,” that may sound great, but you must ask whether this is the
“relative risk” or the “absolute risk.”
Here is an example:
In a clinical trial, one hundred (100) women (the subjects) take a
new drug to see if it reduces the risk of breast cancer, and one
hundred (100) women (the controls) take a placebo (dummy pill).
Assume that after five years, researchers release data showing that
two of the women who took the drug (the subjects) develop breast
cancer and four of the women who took the placebo (the controls)
develop breast cancer.
Based on this data, which “headline” is correct?
“New Miracle Drug Cuts Breast Cancer Risk by 50%!”
“New Drug Results in 2% Drop in Breast Cancer Risk!”
If you said both headlines are correct, you are right. The headlines
represent two different ways to express the data. The first headline
expresses the relative risk reduction — the two women who took the
drug (subjects) and developed breast cancer equal half the number
(50%) of the four women who took the placebo (controls) and developed
The second headline expresses the absolute risk
reduction — 2% of the subjects (2 out of 100) who took the drug
developed breast cancer and 4% of the controls (4 out of 100) who
took the placebo developed breast cancer — an absolute difference of
2% (4% minus 2%).
If you manufactured this new drug, which headline would you prefer?
If you are considering a drug to reduce your risk of breast cancer,
would you be willing to take a drug (particularly if it is associated
with potentially serious side effects) if it reduced your absolute
risk for breast cancer by just 2%?
If you are a health care
provider, do you explain these differences accurately to your
patients so they can make informed decisions? Sorting through the
numbers can be tricky, but can also help put the “headlines” in
Ann's NOTE: Other advocates have noticed, commented and contacted FDA to alert that drug company advertisements sometimes express the benefits of their drug using Relative Risk terms but show the downside, unwanted (side) effects in Absolute terms. That is clearly wrong and misleading. FDA does make the companies remove such ads.
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