It is clear [Variation And The Parallel Structure] that the interaction between variation and the parallel structure of our project models is quite strong. The prediction error caused by ignoring this interaction is significant. We have only two courses of action available to us, if we want useful predictive models. First, we can simply estimate the magnitude of the interaction, thereby including its effect in our predictive models. Second, we can take measures to diminish the magnitude of the interaction effect.
To diminish the magnitude of the interaction effect, component sequences can and should be started earlier. Doing so greatly increases our level of confidence that the outputs of the component sequences will be available for the subsequent assembly task, rather than risking a delay of the assembly task. The degree to which we move early each component sequence is called the component tolerance. This approach is illustrated with our small project, in the next figure.
The first task in each path of the project is an entry task. These, necessarily, are scheduled. Component tolerances tell us how to schedule the start of each entry task. However, this method of scheduling entry tasks is useful if and only if we are working exclusively with a single-project system of resources, i.e. a team of resources that is fully dedicated to a single project and whose team members have no significant commitments to other projects. In a multiproject environment, this method is problematic, as it causes us to schedule the starts of entry tasks as late as possible (ALAP), taking into account the component tolerance. For reasons that are beyond the scope of this writing, the ALAP approach creates problems within a multiproject environment.
Finally, there are other situation where we cannot apply this simple technique directly, even when we are working with a single-project model. We discuss one such situation in the next chapter.
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