Who cares about the critical chain method, amirite? If you’re like me, you assume critical chain project management (CCPM) is an esoteric methodology that doesn’t matter much, unless you’re studying for the PMP exam (which is probably why you’re reading this article.) You figure that, after passing the test, you can promptly forget about CCPM, because you’ll never use it for project planning IRL.
This is what I thought too until I realized that as a practicing project manager, I’ve been using some form of the critical chain method throughout my career—and I didn’t even know it. If you’re new to CCPM or need a refresher, this guide defines the critical chain method and explains how to use it in a real-world example.
What Is The Critical Chain Method?
The critical chain method was developed by Eliyahu M. Goldratt as part of his Theory of Constraints (TOC). It is a technique for managing project timelines that considers resource constraints—whether the people you’re staffing on the project are available to perform the work. Sounds basic, right?
- Define project activities. What work is required to complete the project?
- Estimate task durations. How long does it take to complete each task?
- Calculate buffer. Build in contingencies based on who is executing the work and how much time they have to dedicate to the project.
As a project manager, completing this type of scheduling exercise is likely second nature. You simply didn’t realize that this traditional project management practice had a fancy name: critical chain.
Critical Chain Method vs. Critical Path Method
Our discussion of the critical chain method may have reminded you of another project management process that the Project Management Institute (PMI) tests: the critical path method.
You’ve probably been in a meeting where someone other than the project manager asks which tasks are on the critical path, but I doubt anyone has used the phrase “critical chain” outside of a PMP Bootcamp.
So, what’s the difference?
- The critical path method calculates project duration—the amount of time it takes to complete the tasks as identified in the project plan.
- The critical chain method factors resource availability into the equation.
The table below summarizes the key features of the critical path method vs. the critical chain method.
|Aspect of Method
|Critical Path Method
|Critical Chain Method
|Calculates the time it takes to complete project activities
|Project schedule factors in resource availability
|Buffers are included in the project schedule
|Included as part of activity duration estimates
|Included in bulk at the end of key steps in the project
|Resources may not be 100% available to project tasks
|Resources are 100% available to project tasks on the critical chain; no multitasking
Benefits of the Critical Chain Method
The critical chain method offers several important advantages relative to the critical path method. These benefits include:
- The critical chain methodology allocates resources 100% to the project. That way, if a task finishes early, you can proceed to the next task without encountering bottlenecks.
- The onus falls on the project manager to manage the buffer, rather than on the team to complete their activities within the estimated time frame. In practice, this means:
- If work is running behind schedule, team members do not need to feel pressured about missing a due date. Any late start “delays” simply eat into the buffer.
- If work is running ahead of schedule, team members no longer have any incentive to expand the work to fill the time to complete it (a phenomenon also known as Parkinson’s law.)
- Sequencing tasks without hard and fast due dates reduces the likelihood of student syndrome—when the project team delays starting a task until the last possible moment.
How To Use The Critical Chain Method For Project Control
Fortunately, it’s relatively easy to apply the critical chain project management method. Let’s walk through the steps required.
Define Project Activities
The first step when crafting a project schedule is to understand the activities required for successful project completion. Ask yourself (and your stakeholders!) the following questions:
- What are the desired outputs for this project?
- What work is required to achieve those outputs?
- What are the dependencies (or relationships between tasks)?
- What is the project end date? What is the project start time?
- What are the milestones?
Use the responses to put together a list of sequenced tasks (also known as a work breakdown structure) that aligns with the desired project completion date.
Estimate Activity Duration
Once you’ve identified and sequenced project tasks, estimate the amount of time you expect each task will take to complete. This will vary depending on resource constraints, including the competencies of the person performing the work.
Don’t add any contingencies at this stage; focus instead on generating time estimates that represent the best-case scenario for task finish dates.
This is when it starts to get interesting! Calculating buffer is the way you apply contingencies to your projects when using the critical chain method. Read on to learn more about buffers and the types of buffers involved in calculating project duration.
What Are Buffers In The Critical Chain Method?
Calculating buffer is a fancy way of saying that you are adding extra time to the schedule in case activities take you longer to complete than initially planned. This can be especially important in agile projects when you have limited insight into how to approach the problem you’re trying to solve, and detailed requirements only emerge in the course of development.
The critical chain method accepts that you can try to build in contingencies (or slack) when estimating how long project activities will take. The problem is you’re only human, and humans are notoriously bad at estimating. Instead, it is better to apply time buffers at the end of key project steps to hedge against potential risks in overrunning your estimate.
The critical chain method includes three types of buffers: project, feeding, and resource buffers.
Think of the project buffer as extra time you add at the end of the last project activity to account for any prior delays. This could be the true “drop dead” date for completing the project. Instead of carving up this contingency and distributing it throughout the project, treat it as one block of time and park it at the end of the engagement.
Feeding buffer refers to contingencies you add at the end of a series of non-critical project activities (i.e., the non-critical chain.) The feeding buffer prevents any delays from impacting the critical chain.
I’ve applied a feeding buffer loads of times without even realizing it. A good example is adding a buffer to account for the holiday period when the pace of work slows down and project team members take time off. If the project team is late in turning something around before the holidays, the feeding buffer gives us a grace period to address it before the new year.
The third type of buffer is a resource buffer, which can come in the form of extra people or supplies that remain available to support any activities along the critical chain. With the resource buffer, if a task finishes earlier than planned, we have people standing by to get an early start on the next phase of work.
Critical Chain Method Example
Now that we’ve described how to use the critical chain method for project controls and explained how to calculate buffer, let’s apply what we’ve learned to a real-world example.
Imagine you are working on a study to discover why a high-profile project failed. You are tasked with generating a report of findings that explains what went wrong with this project and how to prevent future projects from failing.
First, we’ll define the project activities. For simplicity, we’ll assume that the project consists of four major tasks:
- Research project documentation
- Conduct interviews
- Write report
- Present findings to the executive team.
Based on your prior experience and conversations with subject matter experts, you’ll estimate how long each of these tasks will take. Let’s assume the following:
|Research project documentation
|Present findings to the executive team
The estimated completion time for this project is ten weeks (2+3+4+1). If we were using the critical path method, we’d apply a 10% contingency to each of these activities to account for potential delays.
In the critical chain method, we instead take our initial estimates (10 weeks) and add the 10% contingency (1 week) to the end of the non-critical chain of activities (in this case, presenting the findings to the executive team is non-critical. You can still write the report without presenting it.) This becomes the feeding buffer.
To calculate the project buffer, take 50% of your total project estimate (5 weeks) and add that to the end of the project.
You add a resource buffer by dedicating people to the project full-time. This means that, if a task finishes early, the team can begin the next one without interruption to the project. For example, if the team is able to consolidate project interviews into 1 week rather than 3 weeks, they can start writing the report sooner.
Looking for more information on project methodologies? Check out our article on project management methodologies and on hybrid project methodologies. and subscribe to the Digital Project Manager newsletter for more sources of great project management content.