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Worked Examples · Example 11

Q.Calculate the standard deviation of the values 5, 10, 25, 30 and 50 by the step-deviation method, dividing the values by the common factor 5.

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With c=5c = 5: x′=1,2,5,6,10x' = 1,2,5,6,10, mean =4.8= 4.8, ∑d′2=50.80\sum d'^2 = 50.80, so σ=10.16×5=15.937\sigma = \sqrt{10.16} \times 5 = 15.937.

Step 1 — Divide the values by the common factor c=5c = 5 (so x′=Xcx' = \dfrac{X}{c}), and find the mean of x′x': xˉ′=1+2+5+6+105=245=4.8\bar{x}' = \dfrac{1 + 2 + 5 + 6 + 10}{5} = \dfrac{24}{5} = 4.8.

Step 2 — Deviations of x′x' from xˉ′\bar{x}' and their squares.

Xx′ = X ÷ 5d′ = x′ − 4.8d′²
51−3.814.44
102−2.87.84
255+0.20.04
306+1.21.44

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