A trial reports that a treatment reduces the risk of an event by 50%. Whether that is a large or a trivial benefit depends entirely on the baseline risk of the patients who were treated. If the baseline risk is 20% (200 events per 1000 patients), a 50% relative risk reduction lowers the risk to 10%, an absolute risk reduction of 10 percentage points, or 100 fewer events per 1000 patients. If the baseline risk is 2% (20 events per 1000), the same 50% relative risk reduction lowers the risk to 1%, an absolute risk reduction of only 1 percentage point, or 10 fewer events per 1000 patients. The relative effect is identical; the absolute effect differs tenfold. The relationship is \(\text{ARR} = \text{baseline risk} \times \text{RRR}\), where ARR is the absolute risk reduction and RRR is the relative risk reduction expressed as a proportion. Because the absolute risk reduction is what determines how many patients must be treated for one to benefit, a relative effect reported without a baseline risk cannot be judged on its own.
Learn MotionHow Doctors Judge Whether a Medical Study Can Be Trusted
Reading the Result: Effect Size, Uncertainty, and Significance
From Relative Risk Reduction to Absolute Risk Reduction
1 / 4
Start with the relative risk reduction slider fixed at fifty percent, and move the baseline risk slider from a low value to a high one. Watch the two bars: the control-group bar grows, and the treated-group bar grows with it, but the gap between them widens. That gap is the absolute risk reduction, and it is what tells you how many patients you must treat to prevent one event. The relative risk reduction never changed, yet the practical benefit did, because the same proportion of a larger risk is a larger absolute number. This is why a relative effect quoted alone, without the baseline risk of the population studied, cannot be judged.
0:00 / 0:00