: <p>

no.of issues  deviation from   square of dev.   percentage
seen              average  <p>

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

The average square deviation is <p>

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

= (50+15+40+45)/100
 
 =  150/100 <p>

=  1.5 <p>

So the SD is equal to the square root of 1.5