The Purpose of Logic Relationships

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The scheduler's primary objective is to identify critical paths. Whatever other purposes the schedule serves, they must not interfere with critical path functionality. CPM practices employed in constructing the schedule must not compromise its ability to accurately model the project. Critical paths must be readily presented with crystalline clarity. To that end, use logic relationships to model actual dependencies. Finish-to-Start relationships are the logical best choice for this. Making this application may not be as easy as you would think. To make it easier, ask if the successor could begin under any circumstances if the predecessor has not completed. If you think it could not, ask why. Are the activities directly related or indirectly related? They are indirectly related if they both depend on something else. That something else could be a common resource. If the dependency is indirect, do not create any link between the two activities. Doing so results in inaccurate Total Float values and obscures actual critical paths.

In the early days of CPM, an arrow diagram was constructed to carefully model all the dependencies in a project. Common scheduling software does not support activity-on-arrow (AOA) networks; but we can still draw them by hand. It would not be feasible to do so for some of today's projects, but AOA analysis is still useful for understanding actual dependencies. If only on a piece of scratch paper or a white board, sketch an AOA diagram to validate the relationships between activities before entering them into a conventional precedence diagram.

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