This method was created in response to the problem of delays in a large number of projects in relation to the plan, where there were higher costs than anticipated and fewer functions than expected.
When designing a project schedule, the most commonly used methods are the critical path method (CPM) and the program evaluation review technique (PERT). These can be used to define an activity, define the sequence of activities, and determine the duration of an activity.
When determining the duration of an individual activity, the project manager tries to secure more time than is actually needed. However, this has several shortcomings that lead to delays in projects.
Specifically, the following three problems occur:
- The task/activity is not executed until the end of its time (student syndrome)
- Tasks/activities are performed in a way that we can finish them by the deadline, although we can finish them earlier (Parkinson's Law)
- We only choose tasks that fit into the allotted time (cherry-picking tasks)
So, the security time we've engaged in performing the activity is overruled. In addition, management often forces people to perform more than one task at a time (sometimes for completely irrelevant reasons). Therefore, people jump from one task to another and extend the time provided by the plan, and by doing so, it takes them much more time to execute an activity than to perform one activity at a time.
In addition to this, people often do not want to report that the task has been completed before time because the next task is planned according to previous experience, meaning that the time scheduled for similar tasks is shortened.
Critical chain project management (CCPM) is trying to solve these problems as follows:
- Critical chain: Defines the critical chain (CC) - that is, the longest chain of dependent tasks. In this case, dependency refers to resources, sharing resources for the same activity, and logical activity dependence. This is the main difference compared to CPM, where only the logical dependency of the activity is considered.
- Time activity assessments: To reduce the time lag associated with excessive safety time, CCPM proposes a reduction in the activity time by 50%.
- Security: CCPM uses security buffers to manage uncertainties during project execution. The safety time that was defined for each task has now been raised to the project level. There are three types of security buffers that ensure the security of project realization:
- Project buffer: This is the time added at the end between the last task and the project completion date. Any delay in the longest chain will consume some of those buffers, but the end date will remain intact. It is recommended that it be half the duration of the total safety time of all activities. Consequently, the total duration of the project is 75% of the original planned.
- Feeding buckets: Delays in CC or long chains can cause project delays, as some of the critical chain vulnerabilities will be delayed. To disable them, add the feed buffers between the last task on the input path and the CC. It is recommended that they be half the time of the safety time of all the incoming route activities.
- Resource buffer: This can be placed next to the CC to provide enough people and skills needed for CC work.
- Priorities: All resources on the project are given clear and defined priorities related to maintaining CC, depending on the particular buffer, as well as information about the project in its entirety. A resource that has more than one open task before completing any task on the path that enters the CC will be assigned to that task that endangers the CC.
- Completion: Resources are encouraged to end the activities in the quickest possible way without compromising quality. Tasks are not left half-finished so that multitasking is avoided. Taking security time adds resources to what is being worked on and eliminates the previously mentioned student syndrome and Parkinson's Law.
- Buffer management: The amount of consumption of each buffer relative to the duration of the project tells us how many delays affect the deadline. If variations of the project are evenly distributed, then the buffer consumption will be linear. The result will be the project completed with the whole exploited buffer. If the buffer consumption is larger than the project's progress, the PM must take corrective action.
- Remaining time: Activities are controlled in relation to the time it takes to complete them (measured by the amount of days) and not according to percentages. This way, they control the buffers and their utilization.