Adding it all up – the final estimate
When you have gathered all of the detailed task estimates, you need to add up the details to a total estimate for the entire project.
As a starting point, you add up all of the expected task efforts. Remember that these are calculated based on your three-point estimates using the formula earlier in this chapter. This total is the total effort most likely needed to complete the project.
Swings and roundabouts
A fairground owner will say: "What you lose on the swings, you gain on the roundabouts." A developer will recognize this: some things take longer (closer to the pessimistic estimate), and some take shorter (closer to the optimistic estimate). However, it is extremely unlikely that everything takes as long as the pessimistic estimate—just as it is extremely unlikely that everything goes swimmingly according to the optimistic estimate.
A statistician will illustrate this fact with a normal distribution curve showing probability or likelihood on the vertical...
