:

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