Exploiting the Plan Using Buffer Management

Measures drive actions that move you toward the goal. In The Haystack Syndrome, Goldratt notes [7],

The first thing that must be clearly defined is the overall purpose of the organization—or, as I prefer to call it, the organization's goal. The second thing is measurements. Not just any measurements, but measurements that will enable us to judge the impact of a local decision on the global goal.

Figure 4.11 illustrates the "cybernetic view" of measures used by Juran. The sensor makes the measure in block 2. An umpire (block 4) compares the output of the process as reported by the sensor to the goal for the process. The umpire makes a decision to cause an action, modifying the process to change output and minimize the gap. This is how all control systems work. This is the intent of project-measurement systems, where the goal includes the technical requirements, cost, and schedule for the project.

In The Haystack Syndrome [7], Goldratt defines data as, "every string of characters that describes something, anything, about our reality." He defines information as "the answer to the question asked." Goldratt suggests that the information system should incorporate the decision.

The improved measurement system for CCPM follows the practice established by Goldratt for production operations. It uses buffers (i.e., time) to measure

Figure 4.11 Dr. Joshep Juran depicts measurement as part of a control process.

task-chain performance. You size the buffers based on the length of the task chain they protect. Buffer sizing uses the uncertainty in the duration of the critical-chain tasks to size the project buffer. Likewise, uncertainty in the duration of the feeding-chain tasks determines the size of each critical-chain feeding buffer. CCPM sets explicit action levels for decisions. The decision levels are in terms of the buffer size, and the percent of the critical chain that has been completed. See Section 6.4.3 for a discussion of buffer trigger points. The buffer is supended into three regions:

1. In the green region, take no action.

2. In the yellow region, assess the problem and plan for action.

3. In the red region, initiate action.

These measures apply to both the project buffer and the critical-chain feeding buffers. Figure 4.12 shows an example of using the buffers.

Project teams monitor the project buffer and each critical-chain feeding buffer at the appropriate time intervals for the project, often daily but at least weekly. For this tool to be fully useful, the buffer monitoring time must be the lesser of at least as frequent as one third of the total buffer time or as frequent as the shortest duration lasts in your project plan. If the buffers are negative (i.e., the latest task on the chain is early relative to the schedule date) or less than one third of the total buffer late (e.g., less than 10 days if the total buffer is 30 days), you do not need to take action. If extended durations penetrate the buffer to the yellow action limit in the project team should plan actions for that chain to accelerate the current or future tasks and recover the buffer. If the task performance penetrates the buffer beyond the red threshold, the project team should take the planned action. Through this mechanism, buffer management provides a unique anticipatory project-management tool with clear decision criteria.

Weekly Buffer History As Of 2/15/2002 9:41:07 AM Project: Puma_Ver_01

Weekly Buffer History As Of 2/15/2002 9:41:07 AM Project: Puma_Ver_01

Figure 4.12 A fever chart plots trends of buffer-penetration versus dynamic-action thresholds.

Project managers update the buffers as often as they need to by simply asking each of the performing tasks how many days they estimate until the completion of their task, or the remaining duration. They do this without pressure or comment on the estimate. They expect these estimates to vary from day to day and for some of the tasks to exceed the original duration estimates. As long as the resources are working on the tasks within the CCPM task-performance paradigm, managers evaluate them positively, regardless of the actual duration.

An enhancement in the use of the buffer for long critical chains is to plot trends for buffer utilization. The buffer measure then becomes, in essence, a control chart and can use similar rules; that is, any penetration of the red zone requires action. Four points trending successively in one direction require action. Most critical-chain software has extended this idea to use a fever chart (Figure 4.12), where the buffer action thresholds may vary over the duration of the project. The idea is that you should not use up the project buffer too early in the project. Trending buffer data preserves the time history of the data and shows the trend of buffer consumption vs. project time. This information helps improve control of the schedule.

Updating the buffers requires that you maintain project status versus your plan in terms of the tasks completed, and estimated remaining duration for incomplete tasks. This is also a useful direct measure of project performance.

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Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

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