Schedule Logic Dependency Objectives

I have long been intrigued by the wide range of reasons why schedulers choose to link two activities together in a predecessor/successor relationship.

Between Hard Logic and Soft Logic

In the early years, when Arrow Diagramming was the only game in town, the linkage between activities was classified as either hard logic or soft logic. That was it!  Hard Logic meant that the linkage was absolutely mandatory – e.g., the roof follows the walls; the walls follow the floor.

Soft Logic, by contrast, always had a degree of subjectivity to it. We may show the work flowing from west to east, for instance. But if something changes in “real life’ to differ from our earliest assumptions, we might chose to proceed from east to west.

Why any of might matter to those who create, maintain, or use Project Schedules can be found in that worn-out adage: “Plan your work … and work your Plan.” The idea is simple enough. Lay out a credible plan … and then follow it!  Over the years a number of penalties and (lesser) rewards have been associated with compliance with the Plan.  So much so, that deviations from the Plan are considered undesirable, or even unacceptable, conduct.

It was in response to the consequences of not following the Plan that the distinction between Hard Logic and Soft Logic emerged. As the thinking went, failure to sustain Soft Logic might be treated more leniently than failure to adhere to Hard Logic. Despite the practicality of the two terms, and their promise of more reasonable responses to deviations between Planned and Actual behavior, the terms shared the same fate as virtually all other important terms in the field of Project Time Management: definitions that, from one authority to the next, suffered inconsistencies, inaccuracies, and general confusion. To this day, one can invoke a hearty debate as to what constitutes Hard Logic and what is, instead, Soft Logic.

Over-Reaction in the Other Direction

In the last dozen or so years we have witnessed a few significant efforts to fully catalog all of the possible ways that activities might be linked together. Some of the more extensive of such studies have identified as many a fourteen ways that activities might be dependent on one another. I happen to think that this is overkill, especially when one considers the otherwise highly-subjective and imprecise nature of all Critical Path Method schedules.

Cognitive Project Management’s Four Restriction Linkage Objectives

Cognitive Project Management contends that the range of reasons for inserting Logic Ties between Activities in a Schedule can be categorized under four Restriction Objectives.  Here they are, as defined by the ICS-Dictionary. I think you will find them fairly self-explanatory.

  • Mandatory Restriction Objective: A Mandatory Restriction is inserted into a Schedule to reflect a mandatory sequencing requirement. For example, a contract may require the completion of Phase II before Phase III commences.
  • Natural Restriction Objective: A Natural Restriction is inserted into a Schedule to reflect a particular sequencing of Activities as required by the very nature of the Work being performed. For instance, it is impossible to paint a wall before the wall is first built.
  • Practical Restriction Objective: A Practical Restriction is inserted into a Schedule to reflect what is deemed by the Project Team (those whose input establishes the content of the Schedule) to be the most practical approach to the Work. An example of a Practical Restriction is the decision to complete floor finishes from the top down in a multi-story structure. Finishing bottom-up would create a situation where dirt and debris of upper floors would be “swept” onto lower, already-completed floors (an impractical scenario).
  • Logistical Restriction Objective: A Logistical Restriction is inserted into a Schedule to reflect logistical considerations of Project Execution, including: the procurement, supply, and maintenance of equipment; the acquisition, deployment, and movement of personnel; the availability of and access to work spaces, and so forth.

To be sure, while there may be four Performance Restriction Objectives, only one of them actually depicts a truly dependent relationship: the Natural Restriction. Who would disagree with the assertion that one cannot paint a wall before the wall is constructed? In this sense, the “Paint Wall” Activity is dependent on the “Construct Wall” Activity.

But when we look at the other three Restriction Objectives, the downstream Activity is not absolutely prevented from performing its Work. Instead, it is simply restricted from proceeding, in deference to an arbitrary agreement among the parties to the Project (and contributors to the Schedule).  The other three are all, to some degree, subjective to the realities at the time of execution.

When you take the word “Dependency” literally, except for Natural Restrictions, are any of the other “downstream” Activities absolutely dependent on the upstream Activities in order to perform their Work? Are they truly being barred from commencing? We think not. And yet all four Restriction Objectives reflect self-imposed commitments made by the members of the Project Team to one another. Once incorporated into the Schedule, these Commitments constitute binding Performance Restrictions.

Finally, a solid understanding of these different Restriction Objectives is important to being able to creating more realistic schedules that reflect prudent expectations.

Revisit Assumptions During Project Execution

We join other leading authorities in the field of Project Time Management who call for the practice of revisiting schedule logic during the course of Project Execution. As you do, you will find the above four Restriction Objectives helpful in determining whether logic between future activities should be changed, and whether deviations between Planned and Actual logic was justified and tolerable or egregious.

What do you think?

2 Responses to Schedule Logic Dependency Objectives

  1. Michael Neal says:

    [C] Great article to read concerning the logic dependent relationships. Explains the reasoning why activites are dependent on on another other than it jus is.

  2. Connie Bremer says:

    In reading about the 4 types of restriction objectives, it does shed light on the subject. I am so pleased that someone is finally addressing the many questions surrounding scheduling. The more we understand, the better we can create, maintain and communicate our schedules.

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