What is the difference between absolute and relative risk?
Keep in mind that the relative risk (or risk ratio) is not the same as an increase in risk. For example, the relative risk of smoking is a factor of ten, but this means that the increase is by a factor of nine. In other words, a 900 percent increase is the same as the relative risk of 10. Similarly, a risk ratio of 2 means a 100 percent increase (the risk gets multiplied by 2).
This is perhaps more evident when the risk increase is small. If we find that a high-cholesterol diet has a risk ratio of 1.65 for rectal cancer in men, this is a 65 percent increase compared to men who have a low-cholesterol diet. The comparison group is considered “1”. If the relative risk is “1”, this means that there is no increased risk.
An important feature of relative risk is that it tells you nothing about the actual risk. This can be very important for evaluating how significant a relative increase might be. A small increase in risk in a large population can result in many deaths. For example, brain tumors are diagnosed in about 6 per 100,000 persons per year, whereas malignant breast cancer is diagnosed in about 134 per 100,000 people. A 10 percentincrease (relative risk of 1.1) in brain tumors means .10 x 6 = .6 new cases per 100,000 people. On the other hand, a 10 percent increase in breast cancer affects 134 per 100,000 people. If the population of the United States is 300 million (which is 3,000 times 100,000), the small increase in brain tumors would result in .6 x 3,000 = 1,800 new cases. In contrast, the same increase of rate in breast cancer would result in 134 x 3,000 = 402,000 new cases, more than 200 times as many.
For another explanation of absolute versus relative risk, take a look at Cancer Research UK’s illustration.