Method 1 – The longer route, with calculations explained
Firstly calculate the average (mean) of your values.
Then take the difference (variance) between each value and the mean.
Then square each variance.
Next, take the mean of the variances.
And finally, the square root of that value gives you the standard deviation of 7.99.
Method 2 – The quick way, using STDEV.P
While Method 1 is great for learning a bit more about how standard deviation works, Method 2 is the in-built Excel function which performs the same calculations.
The function is STDEV.P (or STDEVP if you’re using Excel 2007 or earlier). With the ‘p’ standing for population, this performs a standard deviation for the full population of the selected data. If you are only analysing a sample of the population, you are best to use the STDEV.S function instead, which incorporates some additional variance to account for sample bias.