first we find the mean (average)
(17 + 18 + 19 + 20 + 21) / 5 = 95/5 = 19
now subtract 19 from all values...then square it
17 - 19 = -2...-2^2 = 4
18 - 19 = -1...-1^2 = 1
19 - 19 = 0...0^2 = 0
20 - 19 = 1....1^2 = 1
21 - 19 = 2...2^2 = 4
now take the mean (average) of those
(4 + 1 + 0 + 1 + 4) / 5 = 10/5 = 2 <== this is ur variance
standard deviation is the square root of the variance...
sq rt 2 = 1.414
