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Opens on Chapter 17’s example: Imani’s skate-park survey: 600 people asked, 54% said yes.
Enter your numbers, then press Calculate.
How it works
At its widest the 95% margin of error is about 1 ÷ √n; the fuller form is 1.96 × √(p(1 − p) ÷ n). A share of p is then roughly p ± the margin. The gap between two answers in the same poll wobbles about twice as much, so a lead smaller than twice the margin is too close to call. None of this covers a biased sample or a loaded question.
From the book, Chapter 17. 600 people give a margin of about ±4.1% (the fuller form says 4%), so 54% means about 49.9% to 58.1%. With 1,000 asked the margin is 3.2%; a 4-point lead needs about 6.4. 120 teenagers: ±9.1%. Ten riders: ±31.6%. Every chapter and its calculator →
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