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Critical Chain is focused on a few principles, including: resources fully dedicated to one activity and key resources must be given the appropriate support.
Usng buffers is the better known technique. The idea is to build buffers at the end of chains, to protect and help your key resources.
The buffers are built by taking effort out of non-key activities.The theory is that, dedicating resources will free up extra time that can be re-allocated.
It's worth reading Goldratt's "Critical Chain".
To understand Critical Chain you must have some idea of Theory of Constraints. The crux of the matter is "A chain is as strong as its weakest link". What is the weakest link in project management plan. You cannot take any chances with critical path because any delay on CP is delay in project. Now the question is how to protect the critical path? We have to understand it as the weakest link and to protect it we need to establish buffers to ensure no project gets delayed due to the delay on critical path. If we add a buffer to the critical path in addition to the project duration, that is an increase in original duration which is not acceptable. We have to create this project from within the critical path without adding anything from outside. If we can tax the critical activities and borrow some time from each critical activity, we can put this accumulated time as buffer and restrain the critical activities to their new reduced duration. This project buffer is not in control of those activities from whom it was borrowed and it is the discretion of project manager now if he wants to allow some additional time to one of these activities in trouble, but as a whole a major portion of buffer is secure and is responsible for providing security to the weakest link, our critical path. This will ensure our project will never be delayed and therefore the chain is now string enough. This is called the critical chain.
"extra time that can be re-allocated" or deleted if not used once the task or tasks are completed and the extra buffer wasn't needed.
If you really want to understand Critical Chain I also highly recommend Goldratt's books. Though the topic is a big one and I'm not sure you can reduce it to the top 5 differences between CPM and CC, as a kind of teaser I'll add a little bit information from my point of view ...
Surely the biggest topic is how buffers are used and you already got an excellent explanation on that one. The philosophy here is kind of agile (yes I know that the manifesto does not say anything about what follows): You take the buffer out of every single task and use them to construct buffers that actually help the project protect the critical path and not the task owners protect their behinds. So now lots of tasks will be (mostly just a little) late, but overall the project will be faster and because of the dedicated buffers still reliably hit the due date. So CC values speed over individual due Dates, i.e. tasks are done when they are done, but still has a reliable timeline.
The next thing is that while you burn down (execute) your project along the critical path, you measure how late you currently are as a percentage of the buffer. If you depict % buffer used-up vs. project critical path %done you get the fever chart, which is an easy way to monitor the project health.
Also important in CC is how resources are managed. CC assumes that in every organization there typically is a bottleneck resource that determines the maximum amount of projects the firm can really handle and you need to reduce the work-in-porgress accordingly. If you do not, then there will be lines of tasks waiting in front of the bottleneck with reduced throughput and prolonged project durations. Using the buffers, projects do not compete for this ressource (thereby protecting the efficiency of the resource and prevent urgent re-prioritizations etc.), but just wait until its their time.
Hope this helped...
Hi, Comparing CPM with CCPM is like comparing apples to oranges. CCPM is based on CPM but that's where is similarity/difference ends.
Michael, I think understanding the differences between Critical Path (CPM) and Critical Chain (CCPM) is a key question. I have been implementing, expanding and sustaining the use of Critical Chain for over seventeen years, so I’ve had a lot of practice answering this!
1) Critical Chain BUILDS upon Critical Path (and PERT for that matter)
2) Critical Chain provides a brilliant schedule metric, often called a “fever” or “buffer” chart.
3) Critical Chain improves how to deal with task uncertainty by requiring that task duration estimates OMIT “safety.”
4) That “removed safety” is then aggregated at the end of the project plan (as “buffer”) which is the key to providing the easy-to-understand schedule metric.
5) A Critical Chain is the "Critical Path honoring resource constraints". The longest path is defined only after resource leveling.
Having used both Critical Path and Critical Chain, limiting my projects to CPM would be an evolutionary step backwards. For me, Critical Chain handles task uncertainty better by planning for the best, while maintaining protection for the worst.
In the wiki here you can find information on both CPM and CCPM
I invite you to go and complement the wiki on CCPM, you seem a good candidate to improve on the actual text.
I have the opportunity to take training and practical experience in the field working with Eliyahu M. Goldratt, the "creator" of Critical Chain. The idea is not new. Is outside there from 1956 applied to manufacturing in the field of Operations Research. Critical Chain applied to project/program/portolio (what Mr Goldratt introduced) demands a chain of mind mainly in project/program/portfolio managers. It is the best method to applied when you use Agile for example. Take a look to "Critical Chain" novel to understand it (by the way, I hate the book because is a novel, I did not read it. But I heard is the way to understand the method). Critical Path is a totally different thing due to is tied to scheduling. You can find a lot outside there.
I use CCPM (Exepron tool) in TOCfE program. The schedule starts from the end.
I cut all estimates by 50% and transfer those hours to the buffer. The challenge is manage the buffer consume level rather than activities.
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