Merge Points in Network Logic

By merge point we simply mean a point in the logic where a milestone has two or more predecessors, each with the same finish date. Illustrations of network building blocks shown earlier in this chapter illustrate simple parallel paths joining at a milestone. Such a construction is exactly what we are talking about. Obviously, for all project managers such a logic situation occurs frequently and is really unavoidable entirely; the idea is to avoid as many merging points as possible.

Here is the problem in a nutshell. Let us assume that each of the merging paths is independent. By independent we mean that the performance along one path is not dependent on the performance along the other path. We must be careful here. If there are shared resources between the two paths that are in conflict, whether project staff, special tools, facilities, or other, then the paths are truly not independent. But assuming independence, there is a probability that path A will finish on the milestone date, p(A on time) = p(A), and there is a probability that path B will finish on the milestone date, p(S on time) = p(S). Now, the probability that the milestone into which each of the two paths merge will be achieved on time is the probability that both paths will finish on time. We know from the math of probabilities and from Bayes' Theorem that if A and B are independent, the probability of the milestone finishing on time is the product of the two probabilities:

Now it should be intuitively obvious what the problem is. Both p(A) and p(B) are numbers less than 1. Their product, p(A) * p(B), is even smaller, so the probability of meeting the milestone on time is less than the smallest probability of any of the joining paths. If there are more than two paths joining, the problem is that much greater. Figure 7-10 shows graphically the phenomenon we have been discussing.

Task 1

Task 2

Task 1

Task 2

Date Task 1 and 2 at Date D1 with cumulative probability P3 Figure 7-10: Merge Point Math.

Date Task 1 and 2 at Date D1 with cumulative probability P3 Figure 7-10: Merge Point Math.

Suppose that p(A) = 0.8 and p(S) = 0.7, both pretty high confidence figures. The joint probability of their product is quite simply: 0.8 * 0.7 = 0.56. Obviously, moving down from a confidence of 70% at the least for any single path to only 56% for the milestone is a real move and must be addressed by the project manager. To mitigate risk, the project manager would develop alternate schedule logic that does not require a merging of paths, or the milestone would be isolated with a buffer task.

We have mentioned "shift right" in the discussion of merge point. What does "shift right" refer to? Looking at Figure 7-10, we see that to raise the confidence of the milestone up to the least of the merging paths, in this case 70%, we are going to have to allow for more time. Such a conclusion is really straightforward: more time always raises the confidence in the schedule and provides for a higher probability of completion. But, of course, allowing more time is an extension, to the right, of the schedule. Extending the schedule to the right is the source of the term "shift right." The rule for project managers examining project logic is:

At every merge point of predecessor tasks, think "shift right."

Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

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