Safety is a necessary CYOA mentality that most project managers need projects can finish on time. Trying to balance time management with resources needed is what it is all about and can be summed up in 3 steps; planning, execution, and monitoring.
Two durations are entered for each task: a “best guess,” or 50% probability duration, and a “safe” duration, which should have higher probability of completion.
Resources are then assigned to each task, and the plan is resource leveled using the 50% estimates. The longest sequence of resource-leveled tasks that lead from beginning to end of the project is then identified as the critical chain. The justification for using the 50% estimates is that half of the tasks will finish early and half will finish late, so that the variance over the course of the project should be zero.
Recognizing that tasks are more likely to take more rather than less time due to Parkinson’s Law “Work expands to fill (and often exceed) the time allowed.”, Student syndrome, or other reasons, “buffers” are used to establish dates for deliverables and for monitoring project schedule and financial performance. The “extra” duration of each task on the critical chain—the difference between the “safe” durations and the 50% durations—is gathered together in a buffer at the end of the project. In the same way, buffers are gathered at the end of each sequence of tasks that feed into the critical chain.
When the plan is complete and the project ready to start, the project network is fixed and the buffers size is locked. With no slack in the duration of individual tasks, the resources on the critical chain are exploited by ensuring that they work on the critical chain task and nothing else; multitasking is eliminated. People should be focused on completing the assigned task as quickly as possible, with no distractions.
Because task durations have been planned at the 50% probability duration, there is pressure on the resources to complete critical chain tasks as quickly as possible, overcoming student’s syndrome and Parkinson’s Law.
In some ways is, the greatest advantage of the Critical Chain method. Because individual tasks will vary in duration from the 50% estimate, there is no point in trying to force every task to complete “on time;” estimates can never be perfect. Instead, we monitor the buffers that were created during the planning stage. If the rate of buffer consumption is low, the project is on target. If the rate of consumption is such that there is likely to be little or no buffer at the end of the project, then corrective actions or recovery plans must be developed to recover the loss. When the buffer consumption rate exceeds some critical value (roughly: the rate where all of the buffer may be expected to be consumed before the end of the project, resulting in late completion), then alternative plans need to be implemented.
Critical Chain Project Management – Wikipedia, the free encyclopedia 6/19/2012
Goldratt, Eliyahu M. : Critical Chain, North River Press, Great Barrington, MA. 1997