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Author Topic: On the role of buffers during project execution  (Read 2846 times)
abarua
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« on: March 28, 2012, 08:30:19 PM »

Before a project begins, at the planning stage the role of buffers and how the buffer time is calculated is clear to me. Basically we follow the principles in Critical Chain and setup project ending buffers to absorb variability in the critical path and feeding buffers to absorb variability in the non-critical paths at their junction with the critical path.

Now, once the project begins and execution information is recorded (given Synch with Reality is turned on) the planned dates on the tasks align to reality. Any delay or advancement is reflected in the displayed duration in the buffers in the "PERT" view.

Now let's imagine a diagram with multiple tasks that finishes in an Information Resource, but right before that resource is a Project Buffer. As the project executes, the Project Buffer's displayed time changes to reflect the reduction or even increase of the buffer as the case might be.

Now, if I hit Analyze (sometimes I might have to hit it twice), I've noticed that the buffer duration does not change. And the planned date on the final Resource reflects a date impacted by reality. Meaning, if the first 3 tasks are complete and there is an accumulated delay of 5 days, even though this might be within the Buffer period, the final Resource's planned end date is pushed out 5 days.

This makes me wonder what the planning principles should be:

a) If the critical chain is made up of 10 tasks and the accumulated variability is 20 days, and now that first 5 tasks are completed. During re-planning should we leave the buffer unchanged or update it to only account for the variability of the remaining 5 tasks. In essence should the buffer shrink as the project nears completion such that at the completion of the final step the buffer goes to 0

b) If we're not updating the buffer's duration during re-planning, should we change the planned dates of the task after the buffer if there is still time left on the buffer. So, in the above case if the first 5 tasks have accumulated a delay of 5 days, this reduces the remaining buffer time to 15 days. If I replan, I notice that the buffer reflects this and the buffer's date does not change. Yet dates of downstream resources are modified by 5 additional days. Shouldn't they not be changed unless the buffer is exhausted?

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shortki
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« Reply #1 on: March 30, 2012, 12:39:21 AM »

I have not fully understood the examples with specific amounts of time, but seem to have grasped the essence.

There are two important tasks in project management: planning and tracking. In my program, they are closely related, but they must not be mixed. Planning is done at an early stage of the project, when "rules of the game" are established and a plan is formed, whereas tracking is performed on the stage of project execution. These two tasks are separated by time, and the approved project plan is a "watershed" separating them.

When the initial plan is made, we place controlling time buffers in appropriate spots to focus on problem areas. After all, ideally, upon making a PERT diagram we can let processes take their course. But in reality, on the one hand processes are executed with delays, but on the other hand the project manager usually has a 5-10% resource for "increasing efficiency". A skilled manager, in a timely manner using this resource at the right points of a plan, can avoid serious problems. Therefore, the initial plan should not change when tracking — we ought to control buffers on paths and redistribute the reserves according to the discrepancy between our vision of the "right" course of the project and the actual state of affairs.

Dates of the buffer on the critical path cannot be moved, because in fact, any shift of this date implies the failure of the current project and the start of planning a new project, taking into account the mistakes of the past one. There exists a vicious practice of smooth shifting of terms of the project, but the fact that we do not want to admit a mistake does not make it disappear.

Here the following rule works: as soon as we realize that the project cannot be completed in time, we should start planning of a new project, recognizing the mistakes of the initial plan. The critical buffer in red with expressive zeros makes it possible to conclusively realize that it's time to change something.

« Last Edit: March 30, 2012, 12:40:56 AM by shortki » Logged
xaenah
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« Reply #2 on: October 06, 2023, 06:02:19 PM »

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xaenah
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« Reply #3 on: November 01, 2023, 02:26:35 PM »

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xaenah
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« Reply #4 on: January 01, 2024, 10:18:06 PM »

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