Formula
Variance

Test result − Test average

Deviation from the average <−Test result − Test average
Deviation from the average^2

Deviation from the average squared <− Deviation from the average^2
sum(deviation from the average squared)
sum of squares of deviation from the average <− sum(deviation from the average squared)

Number of data <−length(test result)
sum of squares of deviation from the average squared/number of data
This is VARIANCE

Formula
Standard Deviation

sqrt(variance)