Counts to Rates for a Map: Shading, Cut Points and Tiny Places

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Opens on Chapter 14’s example: library-card sign-ups in Millbrook’s twelve districts, from Old Town (9,800 people) to Marina (40).

Area, count, population (one area per line) e.g. Old Town, 392, 9,800

Cut point 1 per 1,000

Cut point 2

Cut point 3

One dot per

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How it works

Each area’s rate is count ÷ population × 1,000. The shade is set by how many cut points the rate reaches, so moving the cut points moves areas between shades with no change in the data. In a small area one person moves the rate by 1,000 ÷ population, so tiny places top and bottom maps by chance.

From the book, Chapter 14. The town’s rate is 36.9 per 1,000. Marina, with 40 people, tops the map at 75; one person moves its rate by 25, against 0.10 in Old Town. Cut points of 35, 50 and 65 put 1 area in the darkest shade; 26, 31 and 41 put 5. At one dot per 100 sign-ups the town gets 23 dots. Every chapter and its calculator →

Rounding is half up, the way the book rounds. Numbers can be typed as 1,450, $38, 12% or 2.5 million. Everything stays in your browser: no sign-up, nothing stored or sent.