Question

Find the standard deviation of 21, 31, 26, 24, 28, 26

Answer 1

Given the dataset

$x = \{21,\ 31,\ 26,\ 24,\ 28,\ 26\}$

We start by computing the average:

$\overline{x} = \dfrac{21+31+26+24+28+26}{6}=\dfrac{156}{6}=26$

We compute the difference bewteen each element and the average:

$x-\overline{x} = \{-6,\ 5,\ 0,\ -2,\ 2,\ 0\}$

We square those differences:

$(x-\overline{x})^2 = \{36,\ 25,\ 0,\ 4,\ 4,\ 0\}$

And take the average of those squared differences: we sum them

$\displaystyle \sum_{i=1}^n (x-\overline{x})^2=36+25+4+4+0+0=69$

And we divide by the number of elements:

$\displaystyle \sigma^2=\dfrac{\sum_{i=1}^n (x-\overline{x})^2}{n} = \dfrac{69}{6} = 11.5$

Finally, we take the square root of this quantity and we have the standard deviation:

$\displaystyle\sigma = \sqrt{\dfrac{\sum_{i=1}^n (x-\overline{x})^2}{n}} = \sqrt{11.5}\approx 3.39$