[15] Diamond-Shaped Networks

Frequently enough we encounter a logistical network with a diamond structure to it. This happens when a task in the primary sequence provides inputs to two successors, one of which is in the primary sequence and the second is in a component sequence. This is illustrated in the next figure, which shows the primary sequence in red and the component sequence in blue.



The diamond structure doesn’t appear to be a problem, until we calculate the component tolerance and insert it into our model of the network. When we do this, we end up with a significant gap in the primary sequence of the network. This is illustrated in the next figure.

The gap is created by two factors. First, the size of the component tolerance, which is based on the variation associated with the entire component sequence, is large. Second, the precedence dependency between the last task in the component sequence and its predecessor in the primary sequence prevents the component sequence from moving to the left. Consequently, when we include the component tolerances in our model of the project, we end up with a what appears to be a most discomforting gap in the primary sequence.



The knee-jerk response of most project managers today is to force the gap to vanish. Unfortunately, the resulting model ignores completely the strong interaction between variation and the parallel structure. As such, the resulting model is overwhelmingly wrong; it grossly underestimates the duration of the project; and it misleads project managers and decision-makers into making commitments that cannot be met by the resources of the enterprise. But, the resulting picture is strikingly comforting for those who lack any understanding of variation, despite the fact that accuracy relative to duration is destroyed.



There is a solution, of course. Rather than eliminating the gap arbitrarily, we can move the earlier part of the component sequence to the left. Specifically, we move early the portion of the component sequence that precedes the problem dependency. By doing so, we uncouple most of the component sequence from the diamond-shaped feature of the network, and we diminish the magnitude of the interaction effect. This is shown in the next figure.



However, this tactic does not allow us to eliminate the gap entirely. If we did so, we too would be ignoring the interaction between variation and the parallel structure. Instead, our robust project design tactic lets us reduce the magnitude of the interaction, which we model with a smaller but finite gap. The smaller gap (shown in the next figure) provides a correction factor that at this time we can only estimate.



How should we estimate the magnitude of the correction factor? At this writing, the most practical way to estimate it is simply by calculating a component tolerance for the parallel segments that are involved directly in the diamond-shaped structure. This gives us smaller component tolerances, which in turn create a smaller gap. But we can do this only in cases where we can uncouple the earlier segment of the longer component sequence.

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