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Schedule Buffer is a block of time added to a project activity schedule to protect due dates of scheduled tasks and jobs from delays and overlays. Buffers signify safety margins that help manage the impact of uncertainty and variation to a deliverable placed on a schedule. The term is usually used in Critical Chain Project Management (CCPM) methodology.
Defining and setting schedule buffers (buffering) means placing necessary strategic points on a schedule of project activities to ensure success of critical chain tasks. Buffering allows a project team to reach sufficient safety levels and protect project tasks from out-of-schedule durations. Buffering provides a mechanism to push resource-dependent tasks to their successful completion in the shortest possible time period.
The CCPM methodology defines four types of schedule buffers, such as:
- Project Buffer. It is placed on a project schedule between the final scheduled task of the critical chain and the estimated project end-date.
- Feeding Buffer. It is a portion of time when non-critical tasks merge with critical chain tasks. The feeding buffer protects a project from missed completion dates of non-critical chain tasks.
- Resource Buffer. It signifies an amount of working hours required by human resources to complete their critical chain tasks.
- Capacity Buffer. It is used in multi-project environments and project portfolios to plan for on-call resources and cover costs required by the resources to complete two or more projects.
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Although there are others, in this chapter we focus on the following three types of buffers:
■■ Project buffers ■■ Feeding buffers ■■ Resource buffers
Project Buffers
The project buffer is a time buffer placed at the end of the critical chain to protect the overall project schedule. Its size can be calculated as the square root of the sum of the squared differences between the original task duration estimate and the reduced task duration estimate.
Feeding Buffers
The feeding buffer is a time buffer placed at the end of a sequence of tasks that lead into the critical chain. Its size is calculated the same way as the project buffer size.
Resource Buffers
The resource buffer is different than the previous buffers. First, it is not a time buffer. It is a flag, usually placed on the critical chain to alert a resource that it is needed. The flag can be placed at intervals such as one week before the resource is needed, three days before the resource is needed, or one day before the resource is needed. Because it does not contain any time interval, it does not affect the project scheduled completion date. It serves merely to protect the critical chain.
Other Buffers
As we have said, there are several other buffers that we could talk about, but they relate to a multiproject environment and are beyond the scope of this overview chapter. The interested reader should consult Leach's book Critical Chain Project Management. Some of the other pertinent buffers found in Leach's book are as follows:
■■ Capacity constrained buffer. A buffer placed between projects to ensure a specific sequence of projects in a multiproject environment
■■ Cost buffer. A contingency added to a project or sequence of tasks to protect overall cost
■■ Drum buffer. The capacity of a single resource that is scheduled across several projects
Continue reading here: Adaptive Project Framework
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Project Time Management - Project Time Management
Find out the difference between Project Buffer, Feeding Buffer and Resource Buffer
Project Buffer
It is a reserve inserted at end of the project. Any delay in the Longest Path will consume some or all of the buffer, and the completion date will not change.
Feeding Buffers:
It is a buffer that is inserted before the first activity on the Critical Chain. This is to ensure that any tasks feeding into the Critical Chain may not delay the Critical Chain. When a delay does occur in the feeding activities, the feeding buffer is consumed so that Critical Chain is not affected.
Resource Buffers:
These are dummy activities with critical resource assignment, added at one or more places on the Critical Chain with so as to ensure that the resource is available and committed to be utilized if needed.
The age old problems the Project Managers face is exceeding their Project Schedule. This is a general problem that almost always happens in during project management. To solve this, the project managers usually plan for a buffer duration in the project schedule so they don’t exceed their Schedule and sometimes, even with this buffer, they end up getting delayed. These delays are caused by bottlenecks in various phases of the project. Critical Chain Method (CCM) or Critical Chain Project Management (CCPM) extends from Theory of Constrains (TOC) which focuses on identifying the constrains and bottlenecks in the system to improve the throughput. As an outgrowth of TOC, Critical Chain Method also tries to identify and eliminate these constrains and bottlenecks to make sure there are no delays in the project and that it doesn’t extend the project schedule. Critical Chain Project Management methodology focuses on planning and managing projects which give special important to the resources required to execute project tasks. The main advantage of CCM over other traditional Project Management Methodologies is that, Critical Chain Method focuses on getting the project done early and meet the Project Schedule strictly. The critical chain can be defined as “the longest path in the network diagram considering activity interdependence and resource constraints.” Basically critical chain is the longest path in project management which prevents the project from being completed in the shortest time assuming infinite resources are available for the task. The critical chain method is a modified form of Critical path method which considers the availability of resources when making the project schedule. The main advantage the critical chain method has over the Traditional Project Methods like Waterfall is how the uncertainties are managed. In traditional method, tasks are padded with extra time and starting each task as soon as possible and doing parallel activities to solve the uncertainties. In Critical Chain scheduling, uncertainty is primarily managed by estimating the average time duration for a task and scheduling these tasks and leaving out all the unnecessary tasks to save time. Aggregate buffers are placed in the project plan to protect the entire project and the key tasks; and buffer management is done to control the plan. The key tasks are those on which the ultimate duration of the project depends, also known as the Critical Chain. In a Critical Chain Project Management, buffers are used. These buffers are created during the planning to add safety times to tasks within the project so there is a no risk of exceeding the project schedule. These buffers are split into three types, namely Project buffer is placed between the schedule of last task in a project and the project completion date. This “safe” duration buffer acts as a contingency for the project activities. Any delay on the critical chain will consume this buffer, but the project completion date will remain unchanged. If any activity in the project is completed early, then the gain would be added to the Project Buffer. The duration of this buffer is usually 50% of the contingency that was removed from each task estimate. This helps to move uncertainty from each task to the project buffer. Once the project moves into execution phase, this buffer size is fixed and cannot be modified. Any path of activities merging into the critical chain is called a feeding chain. A feeding buffer protects the critical chain against violations in the feeding chain. Basically feeding buffers are added to the non-critical chains so any delay in non-critical chains does not affect the critical chain. Like Project Buffer, feeding buffers are placed between the last activity of a feeding chain (non-critical chain) and the activity on the critical chain which will be added to the project baseline schedule. These feeder buffers are calculated the same way as the project buffers. Resource can be a Person or an equipment. Usually resource buffers are kept alongside the critical chain to make sure they are available when they are required. While the Project Buffer and Project Buffer add “Safe” time so the project is completed within the scheduled time, resource buffer makes sure that the critical resources are available to work on the critical chain activities.What is Critical Chain and Critical Chain Project Management?
1. Project Buffer
2. Feeding Buffer
3. Resource BufferProject Buffer
Feeding Buffer
Resource Buffer