Understanding Schedule Critical Path Drag

Critical activities are all scheduled tasks that cannot be delayed without postponing the project’s completion date. Therefore, these activities are critical; you must consider their progress. But of all your critical activities, which provides the most opportunity for schedule compression?

When considering drag, one likely thinks of a longitudinal force exerted by a fluid, like water, on a solid. This mechanical force or drag is generated by the difference in velocity between the two. If both are motionless, there is no drag. If there is no fluid, there is no drag, i.e., the solid moves in a vacuum. Drag is a force and a vector quantity having both a magnitude and a direction. Its direction is opposite the forward progress of the solid object. This opposition to forward progress may be a clue to the term’s usage in scheduling.

The choice of the term drag in scheduling could have to do with isolating the task that most slows down the project’s progress. But the concept developer coined the term as an acronym play on his last name, Devaux, and the investigation: What time saving is achieved by removing the task from the project?

So, DRAG is an acronym for Devaux’s Removed Activity Gauge. Because only critical tasks have drag, the proper terminology is critical path DRAG. We want to discuss drag in terms of this schedule metric, critical path DRAG on a project, and not necessarily attributes like the physical resisting drag force on a solid.

This article discusses Devaux’s critical path DRAG metric, demonstrates how to find it, and its utility in project scheduling.

Critical Path DRAG

What is Devaux’s critical path DRAG?

Devaux defines drag as the amount of time a critical path activity can be shortened before its drag is reduced to zero (meaning that it is not adding time to the project duration and that another path or paths become just as long (and critical) (Devaux S., p. 107).

The acronym play on words again works out to DRAG. Though the acronym spells DRAG, I do not believe the meaning is necessarily inferring attributes similar to the mechanical fluid-force described above. It is a metric that computes the time savings if an activity is removed from the schedule. This is the term in its positive sense.

Critical path DRAG can be thought of negatively as the time by which each item on the critical path is delaying the project’s end date. Because of the dual positive and negative sense of critical path DRAG, I don’t think the acronym was coined primarily to insinuate resistance to progress; it more properly identifies an opportunity to shorten the project’s duration.

What rules help us find a task’s critical path DRAG?

In a simple network consisting of finish-to-start (FS) relationships, critical path DRAG is determined as follows:

If a critical activity has no parallel activities, its drag equals its duration.
If a critical activity has parallel activities, its drag is the lesser of the two: (1) the total float of the parallel activity with the least total float or (2) its duration.

Demonstration

Our demonstration schedule’s network logic is displayed in Figure 1.

Figure 1

The critical activities in this schedule are A, B, C, D, and E. Non-critical activities F, G, and H have total float values of 15, 5, and 20, respectively. Activities along the critical path have critical drag values, as displayed in Figure 1. Note the network diagram legend on the top right. The critical drag values were determined as follows:

Activities A and E have no parallel activities, so their drag values are equivalent to their original duration values of 10 and 20 days.
Activities B and C are parallel to F and H, which have total float values of 15 and 20. The drag of B is equivalent to the least total float parallel activity F, which has a total float of 15. The original duration of B is 20, so the drag of B is, therefore, 15. The drag of activity C is 5 days, which is the lesser of the parallel activity total float and the original duration of C.
Activity D’s 10-day original duration is greater than G’s 5-day total float. The drag of activity D is, therefore, 5.

So, the drag of activities A, B, C, D, and E is 10, 15, 5, 5, and 20. Your effort to reduce critical drag to optimize the schedule should first focus on Activity E and then on Activity B. These two activities, E and B, have the most significant critical drag.

The rules for determining critical path DRAG differ when schedules include start-to-start (SS) and finish-to-finish (FF) relationships. Schedulers should also consider a task’s lag. Our demonstration was composed solely of FS relationships and no lags.

Utility of Critical Path DRAG

Knowing that tasks E and B from our demonstration have the longest critical path DRAG values informs us that efforts to reduce these two activities have the greatest opportunity to shorten the project’s duration.

So, Devaux’s critical path DRAG, as mentioned above, does not necessarily describe attributes reflective of the mechanical force of a liquid on a solid. It is a metric of schedule compression opportunity. Though different, the utility of each is similar.

Physical fluid-drag measurement tells a naval architect where to focus ship hull redesign; likewise, critical path DRAG computation tells the project manager where to investigate schedule compression efforts. After critical path DRAG computation, those tasks in the project’s network logic with the most Devaux critical path DRAG are reviewed for possible replanning to optimize the schedule.

The critical path DRAG metric in scheduling supports the creation of an efficient schedule. The problem is determining which critical path activity extends the schedule, considering its duration and float of parallel tasks or its resultant drag and the drag of other critical path tasks. This means determining how much each critical task protrudes to the right on the time scale. The critical path DRAG computation provides this insight.

Often, schedulers think the longest-duration critical path task will have the largest critical path DRAG metric value. However, a 20-day duration activity may have a critical path DRAG of 1 day, and a 10-day duration activity may have a critical path DRAG of 10 days, which is not intuitive. What appeared obvious was wrong.

The critical path DRAG metric helps consider which activity on the critical path has the most significant duration or protrusion from its parallel tasks. It is a schedule extension metric. It tells which critical path task adds time to the project, which is a negative, but this locates your best opportunity to shorten the schedule, which is a helpful positive.

And it could support the increase of the project’s expected monetary value (EMV). This is the monetized business value that a project is expected to generate if it includes specific scope and finishes on a specific date (Devaux S., p. 251).

The replanning could include (1) adding resources to speed up the work, (2) rethinking the sequence of steps, or (3) reducing scope, which is a draconian measure.

Summary

The critical path DRAG is an acronym for Devaux Removed Activity Gauge; it indicates how much schedule compression you gain from removing a critical task. Like drag-force locates the best opportunity for hull redesign, critical path DRAG isolates the best potential for schedule replan.

The critical path DRAG value is about making your schedule compression efforts more efficient and having a greater project-shortening success rate. This process is counterintuitive; you would think the longest critical path duration tasks are where to investigate compression efforts. After computing the critical path DRAG, this is often untrue.

Another closely related term to understand is drag cost, which attempts to quantify the cost of a critical activity schedule delay. Drag cost is the reduced expected project profit due to the time a critical path item adds to the project duration (Devaux S., p. 250).

For more in-depth information on Devaux’s critical path DRAG, refer to his book below:

Devaux S. A. (2015), 2^nd Edition, Total Project Control: A Practitioner’s Guide to Managing Projects as Investments, (pp.107, 250, 251), CRC Press.