A Guide to Implementing the Theory of Constraints (TOC)

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Project Buffers

Critical Chain

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A Shorter Critical Chain

On the previous page we noted that any task that can be made parallel, rather than serial, should be moved, and more importantly any task that can be taken from the critical chain and put into a feeding should also be moved.

On that basis our simple plan has one task that could and should be moved.  Here is the plan as it stands.

The task in question is the first task manned by the gray resource.  We can move it up next to task 2.  This moves it from being in-line or serial with task 4 to being parallel with task 4.

Let’s have a look.

If task 1 is moved in parallel with task 4 then the whole project completion time is reduced.  That is a good thing and the objective of the exercise.  But it also gives us two choices of critical chain.  Here is one choice.

Task 1 is on the critical chain and task 4 becomes a new feeding chain.  Here is the other choice.

Task 4 is on the critical chain and task 1 becomes a new feeding chain.

Both choices are valid!

Which one to choose?

If there is a choice it would make more sense to keep task 4 and 2, both manned by the blue resource, on the critical chain.  Here is how the final buffered project would look.

However, the feeding buffer now makes the whole project longer than the buffered critical chain portion and we may then wish to change the critical chain.  However this will only cause task 4 to mirror the current situation. There is no resolution to this phenomenon.  However, don’t forget the whole project is still shorter than the critical chain that we started with when we had task 1 in a serial position before task 4.

This is a nice little illustration of some of the things that can happen with critical chain, don’t lose sight of the overall objective.

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