Find the standard deviation of the first seven natural numbers.

The numbers are 1, 2, 3, 4, 5, 6, 7 with mean 28 ÷ 7 = 4. The squared deviations are 9, 4, 1, 0, 1, 4, 9, adding to 28, so the variance is 28 ÷ 7 = 4 and the standard deviation is √4 = 2. The option 4 is the variance (and also the mean, which makes it doubly tempting), 6 is the range 7 − 1 and 3 is half that range.