In the calculation of variance for optimistic and pessimistic activity duration estimates made at the 95 or 90 percent level, the denominator of the fraction that approximated 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%. Where did the 3.3 and the 2.6 come from?