Critical Chain and Buffer Management

8/2/2019 Critical Chain and Buffer Management

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Andrei Dragota

An I master TI



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 thewell-known and generally accepted resource-constrainedproject scheduling principles. It introduces the idea ofbuffering projects, rather than individual activities, in a

clever way in order to guarantee that the project deadline ismet, and can therefore be considered as a new approach toassure better project control through project and feedingbuffer management.



This presentation gives an overview of the CriticalChain/Buffer Management (CC/BM) approach of projectscheduling and will be briefly explained along thefollowing six steps:

o Step 1. Come up with aggressive estimates

o Step 2. Construct an ALAP schedule

o Step 3. Identify the critical chain

o Step 4. Determine appropriate buffer positions

o Step 5. Determine appropriate buffer sizes

o Step 6. Insert buffers into the schedule



Based on the general idea that the protection of theproject deadline is the primary goal rather than theprotection of each individual project activity, the activitydurations should be set to an aggressive estimate to avoidthat the work of an activity is smoothed out over a longer

duration than really necessary. Goldratt mentions threemain reasons why project activity durations often aresmoothed out over longer durations, as follows:o Murphy’s law: If something can go wrong, it will o Student syndrome: Wait until urgencyo Parkinson’s law: Work expands to fill the allotted time

Aggressive time estimates refer to time estimates wherethe probability of exceedance is relatively large, e.g. the50% percentile.



In contrast to the commonly used earliest startscheduling approach mostly used in traditional projectmanagement and scheduling , it is suggested to scheduleeach project activity as-late-as-possible nearby its

predefined project deadline. This latest start schedule isquite risky since it makes every project activity part of thecritical path which might put, in combination with theaggressive activity time estimates, the project deadline into

danger.



Similar to the critical path concept that distinguishesbetween critical and non-critical activities, a resource-feasible project schedule has a so-called critical chaindefined as the longest chain in the project that determines

the total project duration. The main difference with thecritical path concept is that it takes the limited availabilityof renewable resources into account.



The new idea of the CC/BM technique lies in the cleverpositioning of buffers to protect the project deadlinethrough the use of three types of buffers. Each buffer has aspecific goal and needs to be positioned at the right place to

protect the right part of the project data, as follows:

o Project buffer: A unique and single buffer to protect theproject deadline

o Feeding buffer: Multiple buffers to protect parts of the

critical chain

o Resource buffer: Multiple artificial buffers that act aswarning signals to assure the availability of resources



Sizing buffers as a way to put safety time in the project isa crucial step of the CC/BM approach. Both the project andall the feeding buffers need to be sized appropriately to beable to act as buffers against delay. Although Goldratt

initially proposed to use the simple 50% rule (i.e. the size ofthe buffers is equal to 50% of the duration of the projectactivities it should protect), a more clever way should beused that sizes buffers according to the risk of the activities

it should protect.



Inserting buffers into the project schedule creates abuffered project baseline schedule that can act as a tool tomeasure performance and provides dashboards (i.e. thebuffers) that need to be monitored to trigger corrective

actions.

Critical Chain and Buffer Management

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