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Opens on Chapter 16’s example: the lock that “cuts bike theft by 75%”: from 8 thefts in 1,000 bikes to 2.
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How it works
Relative change compares the change with where it started: (after − before) ÷ before. Absolute change is the plain difference: 6 fewer per 1,000. One divided by the absolute change is how many people (or bikes, or trips) it takes for one of them to be affected. A yearly chance p, over n years, is 1 − (1 − p)ⁿ.
From the book, Chapter 16. 8 in 1,000 to 2 in 1,000 is a 75% cut, but 6 fewer thefts per 1,000 bikes (0.6%): about 167 bikes locked for one fewer theft. 3 to 6 delays in 100 is up 100%, 3 more per 100 flights. Over ten years, 1 in 200 a year is 4.9%; 1 in 400 a year is 2.5%. Every chapter and its calculator →
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