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I've been doing some standard deviation revision but I'm getting confused as different questions have the mean calculated differently.

In some questions, the mean squared is calculated as the actual $mean^2$

so if the mean was $5$ then the mean squared is $25$.

but some others have it calculated where the actual mean is ignored and instead the top half of the fraction for calculating the mean is squared

for example $126^2/12$ rather than $(126/12)^2$

So when is each one used? Is this different for when calculating for frequency tables and lists of data? $$\frac{\sqrt{\sum x^2-n\bar{x}^2}}{n-1}$$

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  • $\begingroup$ When you use each equation, do you get the same answer? There are a variety of 'forms' of the stdev formula, but they should all give the same value (provided you've correctly chosen the sample vs population stdev formula) $\endgroup$ – user237392 Apr 10 '16 at 13:08
  • $\begingroup$ Just checked the formulas again and I was reading it slightly wrong, thanks. $\endgroup$ – simon Apr 10 '16 at 13:09

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