Sizing CC/BM buffers: The cut and paste method

Submitted by Mario Vanhoucke on Thu, 03/01/2012 - 16:11

The Critical Chain/Buffer Management (CC/BM) approach uses project and feeding buffers to add safety time to the project baseline schedule and to guarantee the timely completion of the project with a high probability. Buffers are sized according to the properties of the path or chain feeding those buffers, such as the length of the path, its total variance or the number of activities it contains. This article describes the cut and paste method, which is probably the simplest method to determine the size of the project and feeding buffers and is solely based on the activity duration of the chain merging in the buffer.

Figure 1 displays a project network with 8 activities. The numbers above each node are used to refer to the aggressive activity durations while the label below the node refers to a renewable resource that is required to perform the activity. The renewable resources A, B, C, D and F have an availability of one, while the renewable resource E availability is restricted to two units. The schedule contains three time buffers and one resource buffer. One project buffer is added to protect the critical chain S - 2 - 6 - 8 - E and two feeding buffers FB4-6 and FB7-8 are added to protect the feeding chains 1 - 4 and 3 - 5 - 7, respectively. The resource buffer RB is added to assure that resource B will be available on time to start with activity 6. More information on the determination of these buffers can be found at “Critical Chain/Buffer Management: Adding buffers to a project schedule”.

?Figure 1. The project network with 2 feeding buffers, one resource and one project buffer

Cut and paste method

The first and probably most simplified method to size buffers has been proposed by Eliyahu M. Goldratt in his book “Critical Chain (1997)” and has been labelled by other authors as the “cut and paste method” or the “50% of the chain method”.

buffer size

half of the duration of the longest path in the chain

The buffer is sized as half of the duration of the longest path in the chain and consequently is equal to the following values:

Project buffer: 50% of (6 + 4 + 2) = 6
FB4-6: 50% of (1 + 1) = 1
FB7-8: 50% of (3 + 2 + 1) = 3

Although this sizing method has the advantage of being easy to calculate, it suffers from oversimplification and should therefore be used with care. The presence of activities with long durations will lead to overly large buffers and project durations. Moreover, this method does not take variability of the activities into account, which is often the reason why it is difficult to justify the use of this method. An overview of other buffer sizing methods as an attempt to overcome the simplicity and resulting weaknesses of the cut and paste method is available at “Critical Chain/Buffer Management: Sizing project and feeding buffers”.

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Sizing CC/BM buffers: The cut and paste method