Schedule Margin

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As this is written, comments are proliferating within AACE and NDIA discussion groups about Schedule Margin. What in some quarters is held up as a best practice is plainly not well understood by leaders in the program management and scheduling professions. Nor, it may be suggested, should it be well understood. It is, at best, an emerging practice.

Questions abound about whether Schedule Margin should be funded, or not? Should it be maintained in or out of the performance measurement baseline? How is it derived? Where should it be placed? Do the answers to these questions vary with project circumstances?

Supposedly, Schedule Margin mitigates risks, specifically unknown unknowns. An often repeated rule of thumb is that there should be one month of Schedule Margin for each year of a project. Yet prominent projects are propelled not weeks or months, but years behind schedule, even with the application of Schedule Margin.

Before embracing Schedule Margin as a best practice, the profession needs to actually study it, develop a body of evidence, elaborate its principles, and validate its effects. At this time, Schedule Margin has not earned a place at the table with established practices like CPM.

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The Best Scheduling Software

Some scheduling professionals express strong preferences for scheduling software, even attributing project success or failure to software selection. Others are more tolerant. A scheduling system that makes sense in one industry may not serve another industry very well at all. Well regarded software may exhibit surprising limitations in some applications. The common man's scheduling software may demonstrate astonishing capacity that others cannot readily match.

Although it was not widely used until the advent of computers, practitioners should keep in mind that the Critical Path Method predates them. By today's standards, the early systems were rudimentary; but they did the job. In some respects, former practices were, in fact, superior. Consider that no prominent scheduling software available at this time supports activity-on-arrow planning. We can accomplish pretty much the same analyses using Precedence Diagramming, but many have difficulty eluding the temptations presented by superfluous software features. Thus, unconventional logic relationships and constraint types abound, disabling the Critical Path capability of any software.

The wise practitioner understands that right thinking is more consequential than algorithms, by orders of magnitude, for project success. It is not the tool, but what one does with it, that matters.

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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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Subordination of the Greater to the Lesser

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Developing a critical path schedule is an iterative process. One encounters numerous puzzles and inconsistencies while modeling the many elements that contribute to a complex project. This is not done in isolation. Many stakeholders become participants. They often promote their own agendas, sometimes vigorously. The scheduler may be pressured to accommodate the special circumstances of one element of the project at the expense of another, or of the whole. Well developed "soft skills" are needed to reconcile conflicting objectives. One of the most consequential hazards associated with this environment is the accommodation that subordinates the greater purpose of the critical path schedule to some lesser objective.

Subordination of the greater to the lesser can occur in a variety of ways. One team member may not want to provide the level of detail required to properly model hand-offs to another team. Another, seeking to avoid accountability, declares his contribution is level-of-effort because it is not easily measured. A subcontractor wants to provide inputs in a spreadsheet rather than a network schedule. Another insists on using a different calendar than the standard for the project, which would result in multiple Total Float values on the same path in the schedule. A manager wants reports that reflect poor management practices and would consume limited scheduling resources.

Some of the project's most insidious risks have to do with how the schedule is developed, but they will never show up on the project's risk register. The masterful scheduler understands the long term effects of placing lower level needs over those of the project as a whole. The purpose of the critical path schedule is to manage the project, not to manage its parts uniquely. The scheduling skills required far surpass mere competence.

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Manage Paths, Not Tasks

The Critical Path Method is all about managing paths. Yet, it is a common practice for organizations with critical path schedules to collect and report task-oriented metrics. This is okay if done properly, but it is not a substitute for managing paths. Critical tasks must be considered as part of the critical path, not independently. Non-critical tasks should be considered as part of a meaningful aggregate that can help spot emerging problems.

Total Float is a path-oriented metric. It is also a forward-looking metric. It provides the information needed to affect outcomes where they matter the most. By aggressively attacking a succession of critical paths, project managers are focusing their attention where it is needed most. Critical path is a force-multiplier. Another path-oriented metric is Total Float Trend. This is a derivative of Total Float and not particularly actionable. If you wait for a trend to emerge before you take action, you already missed the boat. On the other hand, you could use Total Float Trend to show the positive effects of your aggressive path management to your boss or the owner. That would be useful!

Task-oriented metrics are usually backward-looking. An example is Baseline Execution Index. It provides a measure how many tasks completed when they were planned to complete, irrespective of Total Float. It can actually encourage counterproductive behaviors, like cherry-picking. A task-oriented metric that is forward-looking is helpful. By comparing Early Finish dates with Baseline Finish dates in the future, the alert manager spots emerging problems before they become critical. The key is to keep your metrics looking forward. You are not going to change the past.

One of the interesting things about subduing the most critical path is that multiple subsidiary paths may be eliminated in the process. Critical Path works. It worked at Hanford Engineer Works. Make it work for you.

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The Other Half of Your Schedule

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The Critical Path Method calculates Early Start and Early Finish dates on the forward pass. Late Start and Late Finish are calculated on the backward pass. The difference between the two is Total Float. The Early Start and Early Finish dates, or frequently, just Start and Finish dates, get plenty of attention. These are the dates reported to management and the owner or contracting agency. Late Start and Late Finish dates usually just lurk in the background, out of sight and out of mind. Planners and schedulers know how they figure into Total Float values, but seldom actually look at them.

Unless you are routinely looking at both sets of dates, you are looking at only half your schedule. Total Float becomes more tangible and meaningful when associated with dates. This is the way people think and work and take time off.

Include Early Start, Early Finish, Late Start and Late Finish dates and Total Float in your standard view and in depictions of your critical paths.

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Early History of Critical Path Method

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Schedulosophy recommends Management of the Hanford Engineer Works in World War II by Harry Thayer. The subtitle of this book is "How the Corps, DuPont and the Metallurgical Laboratory fast tracked the original plutonium works."

Beginning on page 66 is a section titled, "DuPont's Invention of the Critical Path Method (CPM) and Their Use of it at Hanford." It includes a chronology of the period immediately preceding the early stages of CPM development between September 1939 and January 1941. This account relates that CPM had been developed to its final stage by October 1943.

This 224 page paperback was published by the American Society of Civil Engineers in March 1996.

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