In the calculation of variance for optimistic and pessimistic activity duration estimates made at the LO6 95 or 90 percent level, the denominator of the fraction that approximates the standard deviation of the time distribution changes from the traditional

(b a)/6 to (b a)/3.3 for 95 percent and to

(b a)/2.6 for 90 percent. Where did the 3.3 and the 2.6 come from?