Absolute versus Relative Risk

Absolute risk of disease is the chance a patient has of developing the disease over a certain time. For example, a 50 year old male smoker might have a 10% chance of having a MI in the next 10 years. His absolute risk is 10%.

Absolute risk reduction is the difference in absolute risk between an intervention group and a control group (or an exposed and unexposed group). For example, starting our 50 year old smoker on a statin might decrease his 10 year risk of MI from 10% down to 9%. This would be a 1% absolute reduction in risk. Frequently, we express this number as a number needed to treat (NNT). In this case, for every 100 patients started on a statin, we would expect to prevent 1 MI (NNT=100, ignoring harms for the moment).

Relative risk reduction refers to the same difference in risk, but expresses it as a percentage of your original risk. For our 50 year old smoke, a decrease from 10% to 9% risk of MI represents a 10% relative risk reduction (1% divided by his original 10% risk).

In general, absolute risk is much preferred in EBM circles. Relative risk tends to exaggerate the sense of benefit, and therefore is favoured in industry communication. It is important to recognize that a large relative risk may actually be a completely irrelevant absolute risk. For example, imagine I have a 1 in 10,000,000 chance of contracting Ebola living in Canada. A new vaccine might promise a 50% reduction in my chance of catching Ebola, but my current risk is so low that the absolute risk reduction is only 0.0000001%.



Graves RS. Users’ Guides to the Medical Literature: A Manual for Evidence-Based Clinical Practice. Journal of the Medical Library Association. 2002;90(4):483.