Critical path vs. critical chain

Developed by Sophia Lykke Andersen

Abstract

In project management, keeping track of critical project activities is key to manage a schedule. This article attempts to clarify the difference between two well-established phenomena in project scheduling, which are often confused: critical path and critical chain. When activities have to finish before other can start, start before other 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 on this path will cause the whole project to be delayed. From this phenomenon rises the definition of the critical path. 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 critical as well.
Allocating resources, considering additional constraints, and viewing the project schedule from a holistic point of view allows the manager to see chains of criticalities within the project. The critical chain will be the longest path of activities considering activity independence and resource constraints.
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. Furthermore, a glossary explaining the different terms associated with the two phenomena is given and 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.
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 .

Glossary

Activity: ...
Network: ---
Forward pass:
Backwind pass:
Early start:
Late start:
Early finish:
Late finish:
Predecessor:
Successor:
Start-Start: ...
Finish-Finish: ...
Start-Finish: ...
Finish-Start: ...
...

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.
Today... (reference to the programs section)

The methods

Critical path

- Method
- Example

The method of finding the critical path in a network requires a list of the activities with resp. durations, as well as dependencies.
<image of list>
The dependencies are defined and mapped out by identifying the predecessor of each activity. Activities with no predecessors mark the beginning of the project. As seen in the below image ...
<image of network >
When the network is mapped out, the forward pass can be carried out, considering the duration of the activities. The earliest start of each activity is identified and the duration of the project is found. Now the backward pass can be performed, identifying the latest possible start of each activity, in order to finish at the end date of the project.
<image of network with dates of forward and backward pass marked >
The internal float of an activity, also called...?, is the difference between the early start date and the early finish date of an activity. Activities that can latest start the same day as it can earliest start will have an internal float of 0. Activities with a float of zero will be on the critical path, as delays of these activities will delay the whole project.
<image of network with a critical path marked >

Critical chain

- Method
- Example

Programs

A list of the common programs used to calculate the critical path

Discussion

- Differences between the phenomena/methods
- Limitations to each method
- Where is what used and Where to use what ?