This article discusses some common questions about buffers.
FAQ:
What does negative progress mean?
In the "Project Portfolio" tab, you see your list of projects. For the released projects, under column "Performance" you can see two bars. The top one is the project's progress, while the bottom one is the consumption of the project buffer. Sometimes, the progress is showing a negative percentage.
When the project is released the duration of the Critical Chain is considered as the baseline. During the execution of tasks and projects, LYNX tracks the “Current Longest Chain”. As an example: if a task has an estimated duration of 8 days (after “CCPM reduction”) and the first ETTC value is 12 days after starting the task, then 4 days of “negative progress” is calculated: 8 -/- 12 = -/- 4. Normally negative progress mostly occurs in the early stages of a project.
Why does extending the estimated completion date for a milestone deadline date have an significant impact on a buffer?
The project buffer is added to the end of the project to protect the project's deadline. The tasks are expected to be completed within a certain time, and if they are not, you risk not finishing the project before the deadline. This is why the project buffer is there, it allows a total “delay” in the project but still meeting the deadline. If you have consumed buffer, that means the tasks are not completed within the expected time, so you have less of a fallback time to complete the project on time.
When you extend the estimated completion date of a deadline, that means you have more time before the deadline, and thus more project buffer/fallback, so the percentage of consumed buffer decreases. This is the same for milestone (task) buffers, but on a smaller scale. If you have a deadline on a task and you are not progressing or finishing it within the expected time, you will consume buffer. If you increase the date, then you have more time to finish before the deadline, so you have an extra amount of buffer.
When to use or not to use the option “Consume Feeding Buffer”?
The "Consume Feeding Buffer" option in LYNX, which utilizes Critical Chain Project Management (CCPM) and the Theory of Constraints (TOC), is a key feature for managing the buffers that protect the critical chain from variability in task durations. Feeding buffers are inserted before tasks that feed into the critical chain to protect it from delays.
When to Use "Consume Feeding Buffer":
- Variability in Non-Critical Paths: If there's variability in task durations on non-critical paths that might impact the critical chain, use the "Consume Feeding Buffer" option. This allows non-critical tasks to consume the buffer without immediately impacting the critical chain's schedule.
- Controlling Project Schedule: When you need to maintain a controlled schedule and ensure that any delays in non-critical tasks do not directly extend the project's critical path, enabling this option allows for a more robust and realistic schedule.
When Not to Use "Consume Feeding Buffer":
- Critical Chain is already tight: If your project's critical chain is already at its limit and cannot afford any delays, unchecking the "Consume Feeding Buffer" option prevents non-critical tasks from eating into the buffer that is critical for on-time project completion.
- Limited Room for Buffer: In scenarios where there is no room to insert a buffer without extending the critical chain (e.g., the remaining buffer size is zero), it's advisable to not consume the feeding buffer. This ensures that the critical chain is preserved and the project deadline is maintained.
Example Scenario:
If a milestone is showing significant buffer consumption (depicted as a black buffer), it suggests that the allotted buffer for protecting this milestone is being used up. This could be due to the "Consume Feeding Buffer" option being enabled, allowing feeding buffers to extend the duration of the critical chain. In this case, if there is no room left for buffer insertion without impacting the critical chain, you should untick the "Consume Feeding Buffer" box to prevent further consumption and safeguard the project's delivery date.
In conclusion, the decision to use or not use the "Consume Feeding Buffer" option should be based on the specific conditions and constraints of your project. It is a strategic choice to protect the critical chain while managing the inevitable variability in project tasks. Always consider the overall health of the project and the implications on the critical path when adjusting this setting.
Explain what the 3 buffer status bars for each task mean and how to interpret them and their implications?
The buffer status bars on the task list within LYNX represent three types of buffers: project buffer, milestone buffer, and feeding buffer. The presence and order of these bars provide visual cues on the health and priority of tasks and milestones. Specifically:
- Project Buffer: This is the buffer at the end of the project, representing the margin for the overall project timeline.
- Milestone Buffer: When milestones are defined within the project, they get their own buffers. This is the middle bar that appears only when milestone buffers are set.
- Feeding Buffer: These are inserted before tasks that merge into the critical chain, allowing for absorption of delays without impacting the critical path.
When using the "as soon as possible" scheduling engine, adding feeding buffers is optional, as tasks are scheduled from the project start date moving forward. However, for "just in time" scheduling, which is recommended by the Theory of Constraints, tasks are scheduled backward from the project end date, and the use of feeding buffers becomes more critical to protect the critical chain.
Hovering over the buffer bars can provide the order of the buffers if you're unsure of which is which. This feature aids in quickly identifying where buffer protection is applied within your project's schedule.
For CCPM scheduling, you can choose to schedule feeding chains separately from the main scheduling direction, which can be adjusted before and after the project starts, offering flexibility based on project progress and needs. Using "Just in time" is considered the best practice based on the Theory of Constraints methodology.
How to interpret when milestone buffer % for tasks within a milestone are less than the overall milestone buffer ?
If the milestone buffer percentage for individual tasks within a milestone is less than the overall milestone buffer, it suggests that those tasks are consuming less of the buffer than allocated or expected at that point in time. This can be seen as a positive sign, indicating that the tasks are on track or ahead of schedule.
Conversely, if the milestone buffer for a task is high, it may signal that the task is at risk of delay and consuming too much buffer, potentially affecting the project's completion date.
In CCPM, the combination of progress and buffer consumption determines the priority of each task. Tasks on the critical chain with higher buffer consumption become higher priority, as they are closer to affecting the project's delivery date. LYNX automatically prioritizes these tasks, placing them at the top of task lists in "My Activities" or "Active Tasks." When deciding which tasks to start, the priority should be for tasks leading to the overall buffer over tasks leading up to a milestone, as these are more critical to the timely completion of the project. This prioritization ensures that resources are focused on tasks that are most critical to the project's success at any given time.
Related articles:
- Buffer Management - Basics
- How can I add buffers to my Project?
- When are buffers added in a multi-step process?
- The Project and Buffer Statistics panel
- TameFlow Buffer Management
- Relative Buffer Behaviour of Feeding Chains
- Setting the Scene - Lynx Overview
- Include CCPM behavior and buffers into your project
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