no.of issues deviation from square of dev. percentage seen average

0 -1 1 50 1 0 0 20 2 1 1 15 3 2 4 10 4 3 9 5

The average square deviation is

(50x1 + 20x0 + 15x1 + 10x4 + 5x9)/100 =

= (50+15+40+45)/100 = 150/100

= 1.5

So the SD is equal to the square root of 1.5