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 resources available at all times, 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 from a holistic point of view 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, go over 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, a glossary explaining the different terms associated with the two phenomena is given.
The article contains the following topics: Glossary, History, Critical path (Method, Example), Critical chain (Method, Example), and Discussion (Differences, Limitations).

Core difference

A critical path identifies the longest path of activities to be finished in order to end the project .
A critical chain identifies the longest path of activities to be finished in order to end the project also considering resource constraints .

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 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.

Critical chain

Compared to the critical path, the critical chain also considers resource constraints, such as people, equipment, and physical space . Continuing with the example from the previous section, Critical Path, the below table and figure shows the activities and their resourses used:

List of activities and their dependencies and durations

Activity	Predecessor	Successor	Duration	Resourses
A	C	F	2	X
B	C	H	3	Y
C	---	A, B, D	4	X,Y
D	C	I, H	2	W,X
E	J	---	3	W
F	A	T, K	2	Y
G	I	M	3	Y,W
H	B, D	J	4	Z
I	D	G	2	Z
J	K, H	E	2	X
K	F	J	1	X, W
M	T,G	---	3	Y
T	F	M	1	W

Buffers

To try and prevent delays in the project schedule extra time can be worked into the schedule, also called a buffer. The following three types of buffers can be worked into a project network:

Project Buffer: A project buffer is a buffer put in 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. so that delays do not occur.