By: Stephen Devaux, PMP, MSPM
Every chain is as weak as its weakest link, right? So how would you go about making a chain stronger?
If the chain consists of 100 links, and fortifying them would be costly, we wouldn’t want to just randomly select a half dozen to strengthen. We’d like some way to select the weakest ones, those that offer the biggest bang for the buck in terms of creating a stronger chain.
The newly hired general manager of a baseball team looks to identify and improve those positions where the incumbents have been unproductive. A sales & marketing vice president examines those geographic/demographic areas where sales have been lagging. And an aircraft designer seeking more speed attempts to streamline those parts of the plane causing the most air resistance, or “drag”.
Every project is as long as its longest path. Yet critical path theory, and most project management software, effectively tells us only which “links are strongest”: in other words, it identifies non-critical activities that have zero impact on the project’s duration. It quantifies, as total float, the “buffer” to those activities impacting the project’s duration. About the critical path activities that delay project completion, it tells us, literally: “Zero!”
Why does the duration of a project matter? Because shorter projects, just like stronger chains, better hitting shortstops, expanded market segments and faster planes, almost always offer greater value. The shorter project:
- Provides the value of its “product, service or result” sooner;
- Outpaces the competition to market;
- Removes the risk of being late; and
- On projects undertaken to save lives, saves more lives!
Occasionally, projects aren’t made more valuable by being shorter—stronger chains and faster planes aren’t always needed. But such instances are quite rare in the project world. And even then, when unexpected schedule slippage occurs, the project team starts desperately seeking ways to shorten the remaining duration—like the owner who chains his boat to the dock ahead of an impending hurricane now wishes the weakest links were a bit stronger!
Critical path theory, and project management software, needs to tell us not just how far removed from critical a non-critical activity is, but also how much time each critical activity is adding to the project duration. Because that is the information that tells the team where to go to have the greatest impact on shortening the project! Yet the theory, and most software, simply says zero—that the float of each critical path activity is zero.
What the team needs to be told is how much time each critical path activity is contributing to the project’s remaining duration: its critical path drag. And depending on the details of the schedule, a 30D activity on the CP could have just one day of drag, while a 10D activity might have 10D of drag. Figure 1 below shows the drag of each activity in a short network:
Figure 1: A simple network diagram with float and drag computed
In a small and simple network with only finish-to-start (FS) relationships, the “rules” for computing drag are not too complicated:
- If an activity has nothing in parallel, its drag is equal to its duration.
- If an activity has other items in parallel, its drag is whichever is less:
- Its duration, OR
- The total float of the parallel item with the LEAST total float.
In Figure 1:
Activities A and E have nothing in parallel and so have drags equal to their respective durations.
Activity B has a duration of 12D and is parallel with Activities F and G that have floats of 20D and 11D. Therefore Activity B’s drag is 11D.
Activities C and D have durations of 3D and 32D, respectively. But in addition to being parallel with Activities F and G, they are both also parallel with Activity H and its float of 4D. Therefore Activity D is limited to drag of 4D, while Activity C can only be adding three days because that is its duration.
If all networks were as simple as this one, drag computation would be easy even if scheduling software didn’t support it. Unfortunately, larger networks and complex dependencies (start-to-start, finish-to-finish, start-to-finish and lags/leads) make the computation much more difficult. Additionally, as soon as we alter something, all the computations change. For example, if in Figure 1 we were to add resources to Activity D such that its duration changed to 20D, not only would the project duration be compressed by 4D (i.e., Activity D’s drag) to 56D, but the critical path would change and several drag totals would change. To continue the schedule compression process, we would need new computations which, on a large project, would be quite burdensome without the software functionality.
Slowly, software is emerging that computes critical path drag. Spider Project launched such an algorithm in 2009, and enhanced it to compute drag on a resource-leveled schedule in 2014 (yes, resource bottlenecks can also have drag!) In 2015, Boyle Project Consulting created an add-on to Microsoft Project that computes drag. InterPlan Systems has announced that its next release will compute drag. And in August 2016, Elecosoft announced that Version 14 of its Asta Powerproject software, to be launched in October 2016, will compute drag.
But meanwhile, seventeen years after the concept and its computation was explained in the first edition of my 1999 book Total Project Control, the PMBOK® Guide and other standards still make no mention of drag. The whole project management discipline inches away from its most powerful tool, critical path analysis, and toward the often unsystematic scheduling techniques of agile projects, and worse.
And that’s really a shame. Because every chain is as weak as its weakest link. And scheduling has become a weak link due to the fact that critical path theory and software fail to provide the most critical metric: drag, which costs time and money and, sometimes, human lives. And until we identify and quantify those costs, we cannot adequately address them.
For much deeper exploration of the importance of critical path drag, I recommend my books Managing Projects as Investments: Earned Value to Business Value and Total Project Control: A Practitioner’s Guide to Managing Projects as Investments.
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Some Further Reading on Critical Path Drag
“The Drag Efficient: The Missing Quantification of Time on the Critical Path”, Defense AT&L Magazine, Jan-Feb 2012.
“Introduction to the Basics of Scheduling, and Drag as the Metric for Project Delays” by Dr. Tomoichi Sato.
“A Importancia das novas metricas “Drag” e o “Custo Drag” na Analise do Caminho Critico”, written for and published in Brazil by Peter Mello, 22 de fevereiro de 2015. (English translation here.)
“Seventeen Years Later, Doesn’t Critical Path Drag Belong in the 6th Edition of the PMBOK® Guide?”, LinkedIn blog, July 1, 2016.
