People have successfully used a variety of methods to make and control critical chain plans. Initial critical chain projects all used some type of manual method. Keep in mind that we are cautioning against putting too many tasks in a critical chain plan (i.e., a critical chain plan should have no more than a few hundred activities, preferably fewer than 100).
6.6.1 Manual method
The simplest and most commonly used method to manually create a plan is to use the PERT chart format and sticky notes. The procedure follows:
1.Fill out a sticky note for each task, containing the task ID, title, duration (reduced), and controlling resources. (You may want to use color coding to identify the task duration controlling resource.) On the left of the note, indicate the tasks that provide needed input.
2.Lay the notes out on a board or table according to the task logic and following the rough time logic (this is called a time-phased PERT or a time-phased logic diagram).
3.Remove resource contentions.
4.Identify the critical chain.
5.Add sticky notes for the project and feeding buffers.
6.Size the feeding buffers.
7.Calculate the critical chain using a forward pass. Starting with the initial task, write the start times on the lower left of the note and
the completion time (start time plus duration) on the lower right corner.
8.Calculate the feeding paths using a backward pass from where they enter the critical chain.
9.Remove any remaining resource contention and revise the cal- culation.
10.Identify the locations for the resource buffers.
11.Size the resource buffers.
This process is not difficult for projects with 10 to 50 tasks. It gets harder after that.
You may refine the method by cutting out colored paper bars to represent each task. The length of the bar represents task length, and the bar color represents the task duration controlling resource. That simplifies the resource contention steps and subsequent calculation. It obviously requires a little more upfront preparation. Large projects have used this method successfully with over 500 tasks. Using a magnetic scheduling board is another way to implement the same idea.
6.6.2 Critical path software
You can use critical path software to plan and manage critical chain projects. Most software packages have sufficient options to support you in leveling the resources and using late start on the feeding chains. You always start from the same place: with a project logic containing the reduced task times and resource requirements. You should ensure (when necessary) that you have selected the appropriate options to maintain the fixed task duration that you input and that you have selected options to late-start each path. Sometimes, you can do that globally. Other times, you can put constraints on the first task on each path that causes all the downstream tasks to late-start. (You need to experiment and understand what your software does to those options or constraints during resource leveling.)
Most critical path software provides options for the algorithm to perform resource leveling. You can experiment with them. The critical
chain method does not depend on the algorithm you use. It simply requires that the final plan have removed all resource contention within the single project. Usually, you can do resource leveling manually and view the final resource allocations by task.
After initially leveling resources, you must identify the critical chain.
We suggest you add links to the plan to cause the resource leveling to stay in place. You can then remove other constraints that your software may have added to implement resource leveling (e.g., some software adds fixed task start-date constraints to implement resource leveling). If you do add logic connections, you then should be able to calculate the sched- ule and have the critical path equal the critical chain.
Make sure the critical chain you identify really is the constraint of your project. Sometimes an inadvertent logic connection results in tasks on the critical chain that cannot or should not determine the duration of your project. (We call such a connection a mathematical critical path/chain.) Adjust logic or task duration to cause the critical chain to be a legitimate constraint to your project. (Note again that there may be two nearly equal length paths vying for the critical chain. We suggest you choose the one that you feel has higher uncertainty or that makes most use of a potentially capacity-constrained resource.)
The distribution of tasks on the critical chain must provide effective immunity from variation in any one task. There are two simple guidelines for doing that.
1. Make sure the critical chain comprises at least 10 tasks (unless your project is very small).
2. Make sure no single critical chain task comprises more than about 20% of your critical chain or more than 50% of your project buffer.
Next, add the feeding buffers and the project buffer. You add these as tasks, without resource requirements. Remember to tie in the feeding buffers as predecessors to the critical chain task at the point they join the critical chain. Then recheck the resource leveling and make any final adjustments. (Adding the feeding buffers usually requires redoing some amount of resource leveling.)
6.6.3 Critical chain software
Critical chain software automates most or all of the process. Several software packages are currently available, and we understand that most major project management software will be adding the capability over the next year. The most widely available software currently used is ProChain, an add-on to Microsoft Project. Concertois another cur- rently available product.