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Opens on Chapter 19’s example: Millbrook’s twenty cafés: seats against cups sold a day.
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How it works
The correlation r runs from −1 to 1 and says how tightly the dots follow a straight line. The best-fit line is y = a + b × x, with b = Σ(x − x̄)(y − ȳ) ÷ Σ(x − x̄)² and a = ȳ − b × x̄. Leaving each point out in turn shows whether one point is doing all the work. A line says nothing about cause, or about x values outside the data.
From the book, Chapter 19. The cafés give r = 0.93 and a line of about 44 + 4.8 cups per seat. Add one busy six-seat kiosk and r falls to 0.28, with a line of 176 + 1.9 per seat. Every chapter and its calculator →
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