Crital Chain and Critical Path
Critical Chain Project Management (CCPM) is a project management method developed by Eiyahu Goldratt in the 1980:s and first published in the book Critical Chain (ISBN:0884271536). One of the essential ideas is that variance in project duration is inherent to the project environment and it cannot be reduced with efforts to make the individual task more deterministic. At the same time, it is very important to deliver agreed project content, on time and within budget. Elyahu saw that many organizations in their effort to deliver on time tended to start more projects earlier and where caught in a negative spiral of increasing estimates, longer project cycles, more parallel projects, need for more resources and so on. Elyahu decided on a very different solution. He concluded that in a project there are some tasks that are critical and others that are non-critical. If a critical task is delayed the project is delayed. If a non-critical task is delayed there is no effect on the project. By using buffers, it is possible to protect the plan against variance. Buffers should be placed to protect the delivery dates of the project and to protect critical tasks from being delayed by the non-critical tasks.
There are two causes for a task to be critical. Either the task is on the Critical Path or the resource working on the task is also assigned for a task on the critical path. Elyaha understood that since there may be resource constraints delaying the tasks on the critical path these must also be included as critical tasks. Tasks performed by a critical resource may end up being critical since they may delay the project. He called this new chain of critical tasks defining the longest chain of dependent tasks the Critical Chain of the project.
In the plan below the Critical Path is the longest chain of dependent tasks.
In the picture below the Critical Chain is marked red. Note that the critical resource "John_Developer" is levelled, and the critical chain includes the tasks he is assigned to.
Elyahu realized that even with the knowledge that activities on the Critical Chain (priority chain) have precedence over other activities, this would not be enough to ensure that the projects are completed on time. He must come up with a way to protect the critical chain from variation, without the individual estimates having to be larger, and without more time having to be spent on estimation.