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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.
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.
Recently, Stu Ockman had a chance to share some ideas with ENR Deputy Editor, Richard Korman. Here are the results of their conversation: Ten Minutes with Stuart Ockman.
Here are a couple of links to a cautionary tale published last month by the New York Times: Piles of Dirty Secrets Behind a Model ‘Clean Coal’ Project. The interesting part of the Kemper Coal Files Timeline starts on February 27, 2014, with an email from Tom Stevens to Brett Wingo (though you may find earlier documents interesting as well).
Piles of Dirty Secrets is a fascinating account of a whistleblower standing up to a major corporation and doing everything in his power to make sure the project schedule is reasonable. How would you respond in a similar situation? We’d love to hear your thoughts.
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How do you know whether your schedule is good or not? The following are a few tests to see if your CPM schedule is up to par. If you have additional tests that you like, please provide them in the comments. The idea is to collect rules of thumb for identifying good schedules and eliminating bad ones. Here are half a dozen tests to try out on your next schedule:
Test 1: Does the ‘Total Float’ sort or ‘Longest Path’ filter identify a reasonable critical path for the project?
With multiple calendars, the total float/early start sort may not identify the critical path. Some software offers a Longest Path filter to work around this problem. Make sure the longest path is reasonable. Then check the reasonableness of near critical paths. [If you’re using the Longest Path filter, you may have to make a copy of the schedule and start deleting logic ties so that near critical paths show up when the Longest Path filter is rerun]. If the critical path and near critical paths are reasonable, you’re off to a good start.
Test 2: Do any activities have too much float?
Run a total float sort and examine the activities with the most float. Activities with too much float may indicate missing logic ties or logic ties that have been overridden by reporting out-of-sequence progress when updating. Add logic ties, if necessary, to insure that float durations are reasonable and correctly model the current plan.
Test 3: Do any activities have planned durations greater than the update cycle?
Ideally, project activities should be planned at a level of detail so that activity durations are equal to or less than the update cycle [with certain project specific exceptions]. Thus, if a schedule is being updated monthly, planned durations should be 30 calendar days or less. This means that each activity will be in progress for no more than a single update cycle, unless it is behind schedule. By using shorter activities, remaining duration estimates are both easier to make and more accurate, resulting in better status reports for upper management and the project team.
Test 4: Are there any unnecessarily long gaps in workflow when grouping activities by Work Area and sorting by Early Start/Early Finish?
Take a look at an early start/early finish sort grouped by work area, department or phase to get a feel for workflow and resource requirements. Long gaps in an area or phase may indicate less than ideal workflow requiring adjustment of preferential logic ties to create a better plan. In most cases, once work begins in a particular area or on a particular phase of the project, the schedule should allow work to continue uninterrupted in that area or on that phase until it is complete.
Test 5: Are there activities with unnecessary user-assigned constraints?
Since user-assigned constraints override the network logic in calculating early/late dates and float, they should be used sparingly on a project, if at all. A better approach is to use activity durations and network logic to accurately model the project and eliminate constraints. Consider either (1) printing out a constraints listing or (2) running a filter selecting constrained activities. Once you’ve identified all the constraints in the network, you can begin removing them.
Test 6: Are remaining duration estimates accurate?
Too often on a project, remaining duration estimates are automatically generated by reporting activity percents complete at each update. First, make sure that any automatic software link between remaining duration and percent complete is turned off. Next, make sure that every time the schedule is updated, the people responsible for getting the work done provide the remaining durations for activities in progress. Without accurate remaining duration estimates, no downstream dates or contingency times (float) will be accurate, making the schedule a candidate for printing on softer paper.
Try these tests on your project and let us know what you find. And, add your own tips and tricks to the comments below. Good luck and happy scheduling!
by Stuart Ockman
First, a little perspective. These thoughts are in response to articles and talks in the last several years suggesting that part of an expert’s role is to select the methodology that produces the best results for its client.
Here goes: While scheduling of live projects is an art, forensic scheduling is a science. If it were an art, it would not be accepted by Courts and Boards throughout North America and around the globe.
A properly performed forensic schedule analysis is both logical and repeatable. I know this to be true because a few times in my career the expert on each side of a claim reached the same conclusion. So what happened in the couple hundred other disputes? The opposing ‘expert’ either used the wrong methodology or used the right methodology but employed flawed logic.
While there may indeed be nine or more different scheduling methodologies that have been ‘accepted’ over the years as outlined in AACEi’s RP-29, that does not mean that all of these methods are reliable. It just means that the other side did not have an expert able to rebut these approaches by using a reliable methodology.
It is not an expert’s role to ‘methodology shop’ or pick the approach that gives the best result for the client, even if that result is wrong. That’s not just irresponsible but enters the realm of malpractice and risks running afoul of the False Claims Act or incurring other legal sanctions. Instead, an expert must use the best, most accurate methodology available and must use it consistently on all schedule-related claims. Any best methodology must chronologically compare a reasonable plan to what actually happened on the project and adjust the plan to reflect the impact of each controlling delay. Accepted methodologies that meet these criteria are Time Impact Analysis and Windows Analysis. If you’re currently using a different approach on any of your projects, take a look at the literature [one excellent source is Construction Scheduling: Preparation, Liability, and Claims by Wickwire, Driscoll, Hurlbut and Groff] or give me a call. I’d love to chat with you. And, please consider joining us at the College in spreading this and many other messages related to achieving excellence in scheduling throughout the industry and the world.