← A Number's True Size · Lesson 2 of 4
Relative and absolute risk
Suppose something happens to 2 people in every 1,000. If a change makes it 4 in 1,000, the risk has doubled: a 100% rise. That is the relative risk.
The absolute change is 2 more people in every 1,000. Out of a thousand people, 996 are untouched either way.
"Doubles the risk" can be true and still describe a small change. The question that helps is: out of how many, and how many more?
A headline says a habit "doubles the risk" of an illness that affects 2 people in 1,000. What does that mean in counts?
A lesson is kept once you've answered the question and written at least 8 words. Nobody grades it; it stays on this device.