Critical Chain Project Management (CCPM) focuses attention on the few tasks that determine the duration of the project. Projects can be completed 25 to 50% faster, and at less cost. The critical chain is the constraint of a single project. CCPM is most effective in complex environments and with multiple projects that share resources.
Critical Chain Project Management is based on the Theory of Constraints. Drum-Buffer-Rope (DBR) is primarily used to manage production and CCPM is primarily used to manage projects. The distinction between projects and production is blurred. Is building a $200 million aircraft considered production or a project? The only real distinction is how you want to pay attention to the work. DBR monitors the Drum and Buffer, not the process. DBR does not directly track each job or item. CCPM tracks the project as it flows through each resource, so it doesn’t get delayed. High value and importance projects use CCPM to continuously monitor status with confidence.
Critical Chain Project Management (CCPM) uses aggressive task durations with a buffered project commitment. Normal variation in task duration is absorbed by time buffers. Critical Chain focuses attention on the few tasks that impact the actual duration of a project, and it changes behaviors from only meeting due dates to completing tasks quickly. Critical Chain is most effective with complex, multi-project programs that share resources.
The Critical Chain is the longest, task or resource dependent, chain of tasks in a resource loaded and leveled project. Critical Path Method only considers task dependencies. The Critical Path can have parallel tasks requiring the same limited resources.
Critical Path Method
The Critical Path can change during execution. The Critical Chain does not change because it becomes a baseline to measure Critical Chain work completed.
CCPM estimated task durations are based on effort only. No allowances are made for interruptions or distractions of any kind. Each task is considered to have a 50% probability of completing on time. Safety, contingency and allowances for delays (stuff happening) are aggregated at the end of the project (and at the end of feeding chains) in buffers.
Buffers are sized at 50% of the length of the chains they protect. Buffers do not protect individual tasks. Buffers do not belong to management. Buffers can be used by any task and they exist to protect the project commitment. The Project Buffer belongs to the customer.
Critical Chain Project Management
Aggressive tasks are often estimated at 50% of the normal estimate. The Critical Chain is about half the length of the Critical Path. Add in a Project Buffer of 50% of the Critical Chain and the total committed project length is 75% of the Critical Path committed length. Critical Chain projects, even though shorter, average being on time 95+%. Many of them complete in half the Critical Path time.
During project execution, a number of rules and behaviors must be followed –
- Tasks and projects must be prioritized.
- Higher priority tasks have precedence for limited resources.
- A resource is to work diligently on one task until completed.
- Resources are to turn in work when completed (task deliverables are available).
- Tasks must have all inputs available before starting.
- Task status is to be reported as Remaining Duration (effort only).
- Status is collected frequently, usually daily.
Project execution decisions are based on Buffer Management. If a task takes longer than its aggressive planned duration, the buffer will be absorbed. If a task completes in less time than planned, the buffer will recover (grow).
For every project, the percent of Buffer Used (%BU) is compared with the percent of Critical Chain Completed (% CCC). The relative buffer impact (% BU / % CCC) is used to prioritize management’s focus where it is most needed. Resource assignments and special assistance are based on this Buffer Management. During execution, these high priority tasks get management attention. Lower priority (less buffer impacting) tasks receive very little management attention.
Task start and due dates are relatively unimportant in CCPM. What is important is completing the most important work quickly, so the project is completed quickly.