Critical path vs. critical chain

Developed by Sophia Lykke Andersen

Abstract

This article attempts to clarify the difference between two well-established phenomena in project scheduling, which are often confused: critical path and critical chain.

In project management, keeping track of critical project activities is key to managing a schedule. The critical path and critical chain are relevant to project management as these enables the project manager to properly manage the project schedule via e.g. accurate estimation of the project duration, and identification of task dependencies and resource constraints.

When activities have to finish before others can start, start before others can finish, or other relationships, paths of activity dependence form. The longest path of activities related to one another from start to finish of a project will be the most critical, as delays within this path will cause the whole project to be delayed. From this phenomenon, the definition of the critical path rises. In real life, the critical path will only be the most critical considering all available resources, people, materials, space, etc. As soon as resources are limited, other activities or paths of activities can become more critical.
Allocating resources, considering additional constraints, and viewing the project schedule holistically allows the manager to see a chain of criticalities within the project. The critical chain will be the longest path of activities considering activity independence and resource constraints as well.
The article will take the reader through a historical perspective, review the methods of the two phenomena, and provide an example in order to get a more hands-on understanding. In the end, a discussion of the principle differences between the two phenomena will be made, providing the reader with a better understanding of the limitations of each method and when what applies. Furthermore, the article also provides the reader with a useful glossary explaining the different terms associated with the two phenomena and project scheduling in general, as well as a list of useful programs/software for project scheduling.
The article contains the following topics: History, Glossary, Critical path, Critical chain, Software list, Discussion of limitations, and Bibliography with key references and the possibility to read more about the subject.

Core difference

A critical path identifies the longest path of activities to be finished in order to end the project .

They differ in the sense that...

History

The critical path method, also known as CPM, was developed by James E. Kelley from Remington Rand and Morgan R. Walker from DuPont at the end of the 1950s, as a tool against ineffective project planning. They figured that costs could be lowered by making sure that the right project activities were carried out at the right time. Around the same time, the American Navy and Booz Allen Hamilton developed a similar method; the Program Evaluation and Review Technique, also known as PERT. Kelley and Walker published their work in 1959, but back in the 1940s, parts of the technique were already used and contributed to the success of the Manhattan Project.
In the 1960s, larger companies such as Mauchly Associates and Catalytic Construction started to make use of the critical path method, but it was not until the technical revolution of computers that the method could be used without large costs for companies.

The critical chain method, also known as CCPM, was later developed by Dr. Eliyahu M. Goldratt in 1997, building on his theory of constraints. He was very focused on limiting factors such as bottlenecks in order to get a project done on time, which led him to develop this method which does not only consider dependencies between activities of a project but also the resource constraints such as people, equipment, and space.

Glossary

Activity: A task or part of a project, which consumes resources and has a definable beginning and end
Dependency: A relationship between two activities with regard to start or completion relative to each other.
Network: A graphical description of a project plan/schedule, showing the activities and their dependencies.
Forward pass: Definition of the earliest start and finish dates for all activities in a network, made by considering activity durations starting from day 1 of the project going forward.
Backward pass: Definition of the latest start and finish dates for all activities in a network, made by considering activity durations starting from the last of the project going backward.
Early start: The earliest day an activity can start considering the activities prior in the schedule.
Late start: The latest day an activity can start in order for the project to finish on time, considering the activities coming after in the schedule.
Early finish: The earliest day an activity can finish considering the activities prior in the schedule and the activity duration.
Late finish: The latest day an activity can finish in order for the project to finish on time, considering the activities coming after in the schedule and the activity duration
Predecessor: An activity placed prior to the considered activity, defined by the dependency
Successor: An activity placed after the considered activity, defined by the dependency.
Total float: The difference between the early start and the late start, or the early finish and the late finish, of an activity. The total float describes the amount of time the considered activity can be delayed before it causes delays on the whole project schedule.
Finish-start: A relationship where the predecessor has to finish before the successor can start.
Start-start: A relationship where the predecessor has to start before the successor can start.
Start-finish: A relationship where the predecessor has to start before the successor can be finished.
Finish-finish: A relationship where the predecessor has to finish before the successor can finish.
Lag: Lag is a part of a project schedule consuming time but no resources.
Buffer: Extra time or resources added to the schedule to account for unforeseen changes.

Critical path

The method of the critical path, also called the critical path method (CPM), strives to plan out the project in the most optimal way possible, according to the activity dependencies. Buffers and considerations about unforeseen changes are not used, but the actual project performance is tracked and the schedule can be updated accordingly. The health of the project is mostly tied to how well the project follows the schedule. The following section will take the reader through the method of identifying the critical path. The critical path is the longest sequence of dependent activities and it defines the duration of the whole project. It can be found by mapping out the project activities in an activity network diagram. An example is made, making use of an example of an activity-on-node network diagram. Firstly, a list of the activities with resp. durations and dependencies is required. An example of such a list can be found below.

List of activities and their dependencies and durations

Activity	Predecessor	Successor	Duration
A	---	C	2
B	C	G	2
C	A	B, D	4
D	C	E, F	4
E	D	G	3
F	D	G	1
G	B, E, F	---	2

Activities with no predecessors mark the beginning of the project (written as "---" in the table above). The dependencies are defined and mapped out by identifying the predecessor of each activity. From the start node, all the activities with no predecessors are drawn as arrows to each their resp. end node. In this example, the only activity with no predecessor is A. The activities are drawn as boxes with thei activity name (letter) and duration inside. The activity boxes are connected with arrows representing the dependencies between the activities. The below image shows the drawn network according to the example:

A check can be made by going back through the drawn network, making use of each activity's successors. An activity with no successor should be pointing towards the end node of the whole network, like the activity G in the example.
When the network is mapped out the forward pass can be carried out considering the duration of each activity. No activity can start before the day after the last day of its predecessor activity. As A in the example takes 2 days to complete, C cannot start before day no. 3 of the project. The earliest start day of each activity as well as the earliest day it can finish (called early start and early finish resp.) is identified and written on top of the boxes and the duration of the whole project can be found. After that, the backward pass can be performed. This is done by going through the network starting at the finish day and considering the duration of each activity going backward. The latest possible start day for each activity in order to finish by the project's end date is identified and written beneath the boxes for each activity. The below image shows the network with added early start, early finish, late start, and late finish days for each activity.

The difference between the forward and backward pass is called the total float, also called slack. At activity B for instance, the total float is 2, as the difference between 10 and 12, or 11 and 13, is 2. Activities that can latest start the same day as they can earliest start will then have a total float of 0. Activities with a total float of zero will be on the critical path, as delays of these activities will delay the whole project. The below image swos the activity float for each activity (green) and marks the path with 0 total float, and therefore the critical path, with red:

The duration of the whole project is calculated to be 16 days and activities A, C, D, E, F, and G are on the critical path.

Extending the method

The method of the critical path can be extended to larger networks, the definition of work days and weekends coupled with actual dates, and more complicated dependencies between activities.

Relationship:
As for the simplicity of this example, the dependencies have all been kept at what is called a finish-start relationship, where the predecessor has to finish before the successor can start. Other relationships can be start-start, start-finish, and finish-finish relationships.

Lag:
The lag is extra waiting time tied to the relationships between activities, without work or resource impact. In a building project, this could be a waiting time of 28 days for the concrete to dry before the walls can be installed. the relationship would be finish-start, and have a lag of 28 working days.

Critical chain

The method of the critical chain strives to eliminate delays by efficient use of buffers and resource allocation. The health of the project is mostly tied to how easily the resources are consumed. The critical chain in itself is the longest sequence of dependent activities also considering resources. Compared to the critical path, the critical chain manages the project schedule beyond just focusing on dependencies and durations, as it also considers resource constraints, such as people, equipment, and physical space. It enables the project manager to account for unforeseen obstacles and changes and to utilize the available resources as optimally as possible. . The critical path becomes a very essential part of defining the critical chain, as it is one of the three main parts of the critical chain project management (CCPM): Critical path, Feeding chain, and Resource buffers.

Feeding chain

A feeding chain is a chain of activities leading up to the critical path. These activities can also be considered somewhat critical, as delays in these activities can end up causing delays on the critical path, and thereby the whole project. An example of this is activity B from the example. In bigger networks than the one from the example, plenty of activities can be a part of the network, but have very low criticality, as they are not on the critical chain nor even the feeding chain.

Continuing with the example from the section, 'Critical Path', the below table and figure show the same activities and now also their resources needed. The resources are considered assigned 100% to each activity.

List of activities and their dependencies, durations, and resource allocation

Activity	Predecessor	Successor	Duration	Resources
A	---	C	2	X, Y
B	C	G	2	X
C	A	B, D	4	X
D	C	E, F	4	X, Y
E	D	G	3	W, X
F	D	G	1	W, X, Y
G	B, E, F	---	2	X, Y

As seen in the above figure, activity F now has become a part of the critical path as resources are limited. The duration of the whole project is then changed from 14 to 15 days. Before, activity F could be performed simultaneously with activity E, but as both activities E and F need the resources X and W, F cannot start before E has finished. This causes the whole project duration to extend, as it is discovered that the seemingly feeding activity F in reality is a critical activity due to resource constraints.

Buffers

After the critical chain is identified, buffers can be added to the schedule in an attempt to prevent delays. Buffers are extra resources such as time or materials. Three categories of buffers are listed below:

Project Buffer: A project buffer is extra time put at the end of the network, in order to account for possible delays on the critical chain

Feeding buffers: A feeding buffer is buffer time added in between the feeding chain and the critical chain to ensure that delays o the feeding chain are not affecting the critical chain.

Resource buffers: Resource buffers are extra resources such as extra people, space, or equipment set aside in case they are needed on the critical chain, to prevent delays due to resources.

The buffers are placed with respect to the calculated network and the resources available. Possible bottlenecks should be in focus, as well as the resources available.

Limitations

Proper management of the critical path or the critical chain enables the project manager to make an estimation of the project duration, allocate resources effectively before the project start and make changes as the project progresses. The tools are therefore key to successful project execution and will save time and resources in the end, if used correctly. The tools are standard in project management methodologies well established in the project management work field today.

The difference between the critical path and the critical chain lies mostly within the level of elements that are considered. The critical path method relies mostly on the most efficient network of activities, whereas the critical chain method is more focused on resource utilization and the performance of the project. The critical path method can be less flexible, as it relies on rigid scheduling and activity order, whereas the critical chain method focuses more on keeping the resources leveled and keeping flexibility in the start times of the activities.

As the less flexible alternative but more focused on the most optimal schedule, the critical path method is good for specifically defined ventures with few stakeholders . Yet, in the event of unforeseen changes or limitations/relocation of resources, the critical path method does not provide many tools/procedures other than updating the schedule, and delays must be expected.

The critical path method will with its larger focus on resources be more concerned with having resources available when needed, and not so much with having the at most optimal schedule progression. The critical path methods can work for both smaller or larger projects, but its flexible structure works best for projects with larger possibilities of unforeseen challenges, as a fixed project duration will also then be harder to define. Resource constraints could even be minimized before the project starts, are the right choices made in connection to the critical chain management. Mapping out the resources as well as the activities makes collaboration can be done easier and makes it possible to identify bottlenecks. For smaller projects, however, that require a fast turnaround time, additional buffers at additional stages of the project would be required, and the critical chain method would therefore not be as suitable.

Software

Larger project networks are complicated to map out manually, and a program is often used in order to perform the required calculation based on activity durations, dependencies, dates, lag, and other requirements. The softwares/programs listed below are just some of the programs used to calculate project schedules.

• Primavera P6: A software to schedule and manage long-term, complex projects typical for major industries like construction, oil and gas, and others. . A well-established tool for defining the critical path.
• monday.com: A no-code building block application to manage project workflow.
• Microsoft Project
• Asana: A platform for managing multiple teams and multiple projects. Creates and enables easy tracking of workflows.
• ProProfs Project: For managing critical chain activities. Enables allocation of resources. Can create and prioritize tasks within and outside the critical chain.
• ProChain: Manages critical resources and resource allocation, and allows updates on ongoing tasks.
• Aurora CCPM: Manages project schedules in real-time and aligns with project requirements. Interfaces with Primavera P6, Microsoft Project, Oracle, etc.

Additional reading material

... Crashing projects

Conclusion

The article...

References

The following list of resources is used to write this article and can be used to find additional information on the subject.