Calculating overall duration based on probabilities can be very counterintuitive. For example lets say there are few activities in parallel, and lets assume they all have same start data and same most likely duration. Lets say their most likely duration to finish is 10 days, minimum expected duration is 5 days or maximum in 15 days. So they all have a distribution of probability that will look like a triangle, raising from 0 from 5, topping at 10 and decreasing to 15 again. we can all say at once that most likely duration to finish altogether is 10 days but that is wrong. finishing at 10 days is only 0.5x0.5x0.5x0.5 = 12.5% chance. It will take more than 10 days by 100-12.5 = 87.5 % chance. Why? Because even if only 1 activity delays, the whole set will be considered late, just like a whole dinner party is late, when only 1 person is late.