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? $$\frac{\sqrt{\sum x^2-n\bar{x}^2}}{n-1}$$