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Critical chain and buffer management


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Critical chain and buffer management

  1. 1. Andrei DragotaAn I master TI
  2. 2. 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 managementtechnique 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 aclever 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.
  3. 3. 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 estimateso Step 2. Construct an ALAP scheduleo Step 3. Identify the critical chaino Step 4. Determine appropriate buffer positionso Step 5. Determine appropriate buffer sizeso Step 6. Insert buffers into the schedule
  4. 4. 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 longerduration 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 urgency o 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.
  5. 5. 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 itspredefined 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 intodanger.
  6. 6. 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 determinesthe total project duration. The main difference with thecritical path concept is that it takes the limited availabilityof renewable resources into account.
  7. 7. 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 toprotect the right part of the project data, as follows: o Project buffer: A unique and single buffer to protect the project deadline o Feeding buffer: Multiple buffers to protect parts of the critical chain o Resource buffer: Multiple artificial buffers that act as warning signals to assure the availability of resources
  8. 8. 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 Goldrattinitially 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 activitiesit should protect.
  9. 9. 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 correctiveactions.