Critical Chain/Buffer Management: Protecting the schedule against project delays

Submitted by Mario Vanhoucke on Wed, 01/11/2012 - 16:57

The Critical Chain/Buffer Management (CC/BM) approach originally introduced by E. Goldratt in his novel “Critical Chain”, written in 1997 as an application of the “Theory of Constraints”, is a project management technique that introduces some novel ideas on top of the well-known and generally accepted resource-constrained project scheduling principles. It introduces the idea of buffering projects, rather than individual activities, in a clever way in order to guarantee that the project deadline is met, and can therefore be considered as a new approach to assure better project control through project and feeding buffer management.

This article gives an overview of the Critical Chain/Buffer Management (CC/BM) approach of project scheduling and will be briefly explained along the following six steps:

Step 1. Come up with aggressive estimates
Step 2. Construct an as late as possible (ALAP) schedule
Step 3. Identify the critical chain
Step 4. Determine appropriate buffer positions
Step 5. Determine appropriate buffer sizes
Step 6. Insert buffers into the schedule

Details on the various CC/BM steps will be discussed in other articles for which a link is given in between the different steps.

Step 1. Come up with aggressive estimates

Based on the general idea that the protection of the project deadline is the primary goal rather than the protection of each individual project activity, the activity durations should be set to an aggressive estimate to avoid that the work of an activity is smoothed out over a longer duration than really necessary. Goldratt mentions three main reasons why project activity durations often are smoothed out over longer durations, as follows:

Murphy’s law: If something can go wrong, it will
Student syndrome: Wait until urgency
Parkinson’s law: Work expands to fill the allotted time

Aggressive time estimates refer to time estimates where the probability of exceedance is relatively large, e.g. the 50% percentile. More details on these time estimates are given in ”Aggressive activity time estimates: protecting against activity delays”.

Step 2. Construct an ALAP schedule

In contrast to the commonly used earliest start scheduling approach mostly used in traditional project management and scheduling (see ”Scheduling projects: How to determine the critical path using activity slack calculations?”), it is suggested to schedule each project activity as-late-as-possible nearby its predefined project deadline. This latest start schedule is quite risky since it makes every project activity part of the critical path which might put, in combination with the aggressive activity time estimates, the project deadline into danger. However, this approach also offers certain advantages which are illustrated and discussed in ”Critical Chain/Buffer Management: (Dis-)advantages of scheduling projects as-late-as-possible”.

Step 3. Identify the critical chain

Similar to the critical path concept that distinguishes between critical and non-critical activities, a resource-feasible project schedule has a so-called critical chain defined as the longest chain in the project that determines the total project duration. The main difference with the critical path concept is that it takes the limited availability of renewable resources into account (see ”The critical path or the critical chain?: The difference caused by resources”).

Step 4. Determine appropriate buffer positions

The new idea of the CC/BM technique lies in the clever positioning of buffers to protect the project deadline through the use of three types of buffers. Each buffer has a specific goal and needs to be positioned at the right place to protect the right part of the project data, as follows:

Project buffer: A unique and single buffer to protect the project deadline
Feeding buffer: Multiple buffers to protect parts of the critical chain
Resource buffer: Multiple artificial buffers that act as warning signals to assure the availability of resources

More information and an illustration of the positioning of buffers can be found in “Critical Chain/Buffer Management: Adding buffers to a project schedule”.

Step 5. Determine appropriate buffer sizes

Sizing buffers as a way to put safety time in the project is a crucial step of the CC/BM approach. Both the project and all the feeding buffers need to be sized appropriately to be able to act as buffers against delay. Although Goldratt initially proposed to use the simple 50% rule (i.e. the size of the buffers is equal to 50% of the duration of the project activities it should protect), a more clever way should be used that sizes buffers according to the risk of the activities it should protect. More information on buffer size techniques can be found in “Critical Chain/Buffer Management: Sizing project and feeding buffers”.

Step 6. Insert buffers into the schedule

Inserting buffers into the project schedule creates a buffered project baseline schedule that can act as a tool to measure performance and provides dashboards (i.e. the buffers) that need to be monitored to trigger corrective actions. More information can be found in ”Critical Chain/Buffer Management: Inserting buffers in a resource-constrained schedule”.

PM Knowledge Center

Critical Chain/Buffer Management: Protecting the schedule against project delays