You’ve heard me talk about critical paths and the critical path method, CPM. You’ve heard me rant about it and rave. As I get older and see more of this, I’m doubling down. I was writing a post about critical chain and what I call a critical flow path, and I read something telling: critical chain project management is different and useful. The application of CCPM has been credited with achieving projects 10 to 50% faster and/or cheaper than traditional methods like CPM, PERT, and Gantt, developed from 1910 to the 1950s.
Short answer: A critical path is a projection that ignores trade flow and buffers. Critical chain project management focuses on resources and flow, with buffers, and treats the schedule as a complex system. In construction, takt planning lines up with critical chain through a critical flow path: phases with a sequence, a line of balance, and a buffer, networked to a project buffer, with buffers in the supply chain too.
Where Did CPM Come From?
The more I look into the development of CPM, the clearer it is that this wasn’t a think tank, a wonderful technological invention, or based on science. It was two guys, one from DuPont, coming together, coming up with a system, and making it popular. I can’t understand why people defend it so much, but I’m not going to get too negative. You know how I feel about it.
In a CPM schedule, whether P6, Microsoft Project, or whatever software, you have a work breakdown structure, usually by deliverable and very rarely by location. Some people are annoyed at doing a WBS by location, but if you’re going to do CPM, you’ve got to do it by location. Time is on the top and the deliverable is on the left. Each activity gets at least a predecessor and a successor. Then the CPM algorithm does a forward and backward pass and finds the longest path with zero float.
In most cases, not all, what you end up with is a schedule with no buffer or contingency, with activities on a critical path being focused on without considering the other impacts, other float paths, or the fact that the schedule is a complex system. A critical path is defined as the sequence of dependent tasks that form the longest path, and if any of them are delayed, the project is delayed. It sounds cool. But it’s not a thing. We don’t have them. They’re like gargoyles: not real, and not even nice to look at. See You Don’t Have a Critical Path!.
What Is Critical Chain Project Management?
There’s a book called Critical Chain by Eli Goldratt. I’m a huge fan. CCPM is a method of planning and managing projects that emphasizes the resources, the people, the equipment, and the physical space required to execute project tasks in a flow with buffers, understanding that the schedule represents a complex system. There isn’t just one critical path.
What I love is that it recognizes buffers everywhere. Phases within a schedule should have buffers. The overall project should have a buffer at the end. Feeding activities should have feeding buffers. You should have capacity buffers for the workforce and material buffers, some way for the equipment, tools, workers, or materials to absorb interruptions and variation.
I’m almost done reading it, and I’m like, where have you been all my life? The takt production system lines up exactly with it. If you haven’t read it, just go read it.
Why Are Critical Paths a Problem?
Here is what I posted on LinkedIn. I learned about critical paths early in my career, like many of you. The term is well-known, but being well-known doesn’t make it right. Throughout history, many popular things were absurd, like installing carpets in airports, building houses with asbestos, and bloodletting patients as a default treatment. Were they popular? Yes. Advisable? No. Are we going to keep believing in the critical path because it’s popular and taught, or go back to reason, common sense, and observed results? Critical paths:
- Are projections and will constantly change
- Ignore trade flow and encourage overburdening people
- Are used by owners to reject delay claims and shorten project durations to unhealthy levels
- Are mostly used without schedule contingency and lead to compressed schedules
- Put projects at risk by making them more unstable and unpredictable
- Ignore that construction projects are complex systems, focusing on a myopic measurement over important information
- Provide a false sense of security, because you can measure a critical path and be completely wrong
- Are usually formatted time by deliverable, which is only two-dimensional
I can tell you how people will behave on your project based on what you measure. The critical path negates most production principles, and when people use it as a measurement, they make decisions that counteract production.
How Should Construction Projects Be Built and Analyzed?
Applied to construction, the main differences with critical chain are a focus on trade flow and workflow, buffers in the project schedule, and buffers in the supply chain. I’ve found takt planning, steering, and control to be a great scheduling method that aligns with critical chain concepts.
| Element | Critical flow path approach |
|---|---|
| Format | Time on the top, location or the value-receiving unit on the left |
| Organization | Phases and zones |
| Each phase | A sequence duration, repetitive sequences by zone, a line of balance that fits within a mathematical range, and a buffer at the end to absorb risk |
| Project buffer | Most of the buffer at the end of the project |
| Supply chain | Buffers between material lead times and the activity start |
What Does a Critical Flow Path Look Like?
Take a three-phase takt plan. Sequence, line of balance, and buffer for phase one. Interconnection. Sequence, line of balance, and buffer for phase two. Interconnection. Sequence, line of balance, and buffer for phase three. Then the project end, with the project buffer. Each phase buffer is sized in direct response to that phase’s risks, and the sequences are pull planned, coordinated, and optimized.
The zones can be measured with a key indicator Kevin Rice invented at Elevate called Realized Flow Potential. It’s a formula, and when it’s within a certain range, you know the phase is optimized. Material lead times get material inventory buffers, so you sandwich activities between buffers to absorb variation. Those connections are what I call a critical flow path. One of these days, I want us to stop talking about critical paths and start talking about critical flow paths. See Buffers Create Flow.
Why Am I Telling You This?
I’m making no money from telling you this. I’m not selling software or services. I’m posting this because I love you. If you hear negative voices, follow the money and see if dissenters are paid to dissent. And follow the change pattern: every human being, especially me, says this is not relevant, not needed, impractical, or incorrect when we hear something new. Research it, pray about it, or experiment with it. Five years from now, I predict the industry will understand the fallacy of the critical path.
A newer horizontal superintendent wrote that the books and videos help, but there’s so much to learn it’s hard to prioritize. A constant, steady drip of learning, reading, reflecting, and listening is a super good way to educate ourselves in a busy industry.
So here is the challenge. Look at your current schedule. Find where your phase buffers, project buffer, and material buffers are. If you can’t find them, start there.
Bottom line: Use buffers in your system, protect trade flow, use buffers in your supply chain, and start talking about critical flow paths.
If your project needs superintendent coaching, project support, or leadership development, Elevate Construction can help your field teams stabilize, schedule, and flow.
On we go.
FAQ
What is critical chain project management?
CCPM is a method of planning and managing projects that emphasizes the resources, the people, the equipment, and the physical space required to execute project tasks in a flow with buffers, understanding that the schedule represents a complex system. It comes from Eli Goldratt’s book Critical Chain.
What is a critical flow path?
It’s the chain of networked takt phases, each with a sequence, a line of balance, and a buffer sized to that phase’s risks, connected through to a project buffer at the end, with material inventory buffers in the supply chain. Each piece can be analyzed to be within proper ranges.
What is wrong with the critical path?
Critical paths are projections that constantly change. They ignore trade flow, are used to reject delay claims and shorten durations to unhealthy levels, are mostly used without contingency, make projects less stable, ignore that projects are complex systems, give a false sense of security, and are usually formatted only by time and deliverable.
People Also Read:
Goldratt’s Rules of Flow
Buffers Are NOT Shared Float
The Biggest Advantage of Takt Planning Over CPM
Drum Buffer Rope: Protecting the Bottleneck
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-Takt Virtual Training: (Takt Virtual Training)
-Check out our YouTube channel for more info: (Elevate Construction on YouTube)
-Listen to the Elevate Construction podcast: (Elevate Construction Podcast)
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-The Takt Book: (The Takt Book on Amazon)
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