The CPM Schedule as a Multi-Dimensional Model

The CPM Schedule as a Multi-Dimensional Model

Throughout CPM Mechanics we have been advised never to lose sight of the distinction between the Project Execution itself and the model of that Project Execution, the Project Schedule. We were told that Activities in the Schedule represent Actions in the Project. We were told that Logic Ties which connect Activities in the Schedule actually represent the Interdependency Relationships between those human Actions on the Project.

The point to be made is that it is all too easy (and, sadly, too common) to conflate the very real Project with the superficial and virtual Project Model. When we stare at a CPM schedule, we must use our imagination to visualize the Project that the Schedule seeks to model. One way to accomplish this is by taking the additional mental step of appreciating the distinct informational nuances of Schedule content within different Schedule Segments.

Why Schedule Tense is So Important to Understanding Project Schedules

In one of the book’s final chapters we learn about something called Schedule Tense, that Schedules can and mostly do reflect three different Project Temporal States: past, present, and future. We learn that Schedule Segments in the Project Model correspond to Project Performance Periods out on the Project.

I thought that a good use of this last blog posting (in the CPM Mechanics blog series) would be to consider how the meaning of various Primary Schedule Elements might differ, depending on the Schedule Segment within which they appear. For the following discussion, please refer to this table:

Schedule Element Past Present (Imminent) Future
Activity Recorded Actions Opportunity Actions Possibility Actions
Logic Ties (Actual Restrictions) Coordination (internal) Collaboration, Cooperation
Dates Historical Facts Commitment (Current) Intention
Date Constraint (Actual Restriction) Coordination (external) (Current) Expectation
Duration (Actual Requirement) Priority and Autonomy Timeframes (Time Holders)


Different Meanings of Activity: Let us start with the most basic element of any schedule, the Activity. We know that Activities in a Schedule represent Actions on the Project. But is there more to recognize about Activities, with respect to the different Schedule Segments?

  • Past Segment Activities found in the Past Segment represent work that was performed, what we might call Recorded Actions.
  • Present-Imminent Segment Activities found in the short-ranged Imminent Period of the Present Segment can be appreciated as a listing of opportunities for Action that face the Project Team. Viewed this way, we would spend sufficient time in discussions to discover ways to capitalize on those opportunities.
  • Future Segment By contrast, Activities in the Future Segment only describe the possibility of scope performance. Whereas there is little uncertainty that Present Period Actions will be performed, Future Segment Activities clearly state what is not only possible, but reasonably expected to occur.

Different Meanings of Logic Ties: As a generalization, Logic Ties in a Project Schedule model corresponding Relationships on the Project itself. But when we look closer, at the Schedule Segment level, we see important distinctions.

  • Past Segment The truth is that the Logic Ties that we see in the Past Segment of the typical Project Schedule are fairly useless and not to be trusted. I say this because, once the Activities have been statused as complete, the Logic Ties between the Activities are not adjusted to reflect the true Relationship between the corresponding Actions on the Project. I challenge you to find any Best Practices guidelines that call for adjusting the Logic Ties to reflect how the Activities in the Schedule were actually interrelated. And that is why I put the words “Actual Restrictions” in parentheses: they are simply not recorded.
  • Present-Imminent Segment As for Present Segment Logic Ties, these reflect the most current thinking of the Project Team as to how the Imminent Period work will be performed. In other words, it expresses the coordination plans of the Project Team, but only of the work to be performed by Project Team (internal).
  • Future Segment As we turn our focus to the Future Segment, Logic Ties become absolute, as they express the collaborative and cooperative intentions of the Project Team.

Different Meanings of Activity Dates: Activity Dates have different names, depending on the Schedule Segment.  Present and Future Segment Activity Dates are calculated, whereas Past Segment Dates are actual.

  • Past Segment Activity Dates represent historical facts. They account for when corresponding Actions on the Project actually occurred. It is a best practice to record the Actual Dates for Activities that are reported started and/or completed.
  • Present-Imminent Segment Calculated Dates for the Imminent Period represent solid, take-it-to-the-bank commitments among the Project Team members.
  • Future Segment Contrast that with Future Segment Dates which are, at best, a reflection of the best intentions of the Project Team. The word “current” is set off in parentheses because it is not a common practice to revisit the details of the Future Schedule Segment, on the occasion of each Schedule Update. ICS-Research recommends that the Schedule be revisited for confirmation of continued viability, but this is hardly a standard practice in the construction industry. In fact, most construction contracts discourage (and some even prohibit) changes to even the most minute details of the Project Schedule without prior Owner consent.

Different Meanings of Date Constraints: Imposed Date Constraints have the effect of modifying date calculations, pursuant to the functionality of the specific Date Constraint.

  • Past Segment Date Constraints become dormant, or disappear, once the Activities to which they are attached are completed. It would be an interesting enhancement to our Project Time Management processes if we were to note any Date Constraints that actually affected the performance of Activities. Since this is not a common practice, I have put it in parentheses.
  • Present-Imminent Segment In the Present Segment, Date Constraints are included for coordinative purposes, to more specifically and thoroughly describe any agreed-upon pacing factors.
  • Future Segment Date Constraints that appear in the Future Segment are placed there to reflect the expectations of the Project Team with respect to some future event or circumstance.

Different Meanings of Activity Durations: Activity Durations have very different meanings, depending on the Schedule Segment

  • Past Segment Activity Durations, if we would take the time and effort to calculate the amount of time actually required to perform the work, could be a very useful mode of actual performance. But Actual Durations are rarely calculated or captured.
  • Present-Imminent Segment Durations in this Schedule Segment are the most important and essential elements in the Schedule when it comes to the Project Team’s ability to prioritize its work in the near term. It also helps clarify who has “rights” to specific work areas, and when those area are “theirs alone.”
  • Future Segment In the world of publishing there is this thing called the “place-holder,” which is an empty box drawn on the document layout sheets to hold a place for a future diagram or image to be “dropped in” once developed. Likewise, the Durations of Activities in the Future Segment can be thought of as time-holders; we call them Timeframes.

A Monolithic Perspective is Over-Simplistic

I hope you enjoyed the above discussion. The Take Away is that a Schedule is not this monolithic document; it is a fractal. We should hardly be surprised to read this. Across the blogs of this series we have come to realize that a Project is a fractal, as is Project Management and Project Time Management.

Now we see that the Project Schedule, too, has many facets to it, and each facet has its own attributes and limitations. The Critical Path Method is just a technology, nothing more. What we do with it is a very different, and much larger, discussion — which will be considered in sufficient depth in other ICS-Institute Courses, books, and blogs. See you there!

1 Responses to The CPM Schedule as a Multi-Dimensional Model

  1. Zach Reed says:

    A good reminder about the realities of the schedule. We must understand the “technology” in order to know how to manage using the technology.

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