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3 Exploiting the plan using buffer management 117

3 Exploiting the plan using buffer management 117

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118

Critical Chain Project Management

(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. That is
how all control systems work and is the intent of project measurement
systems, in which the goal includes the technical requirements, cost, and
schedule for the project.
In The Haystack Syndrome, Dr. Goldratt defines data as “every string of
characters that describes something, anything, about our reality” [4]. He
defines information as “the answer to the question asked.” Dr. Goldratt
suggests that the information system should incorporate the decision.
The improved measurement system for CCPM follows the practice
established by Dr. Goldratt for production operations. It uses buffers (i.e.,
time) to measure task chain performance. Recall that the end of the project buffer is a fixed date: the project delivery date. For buffer management
purposes, you also fix the ends of the feeding buffers. You determine
project buffer penetration by asking people working on tasks, “When will
you be done?” That allows you to project forward using the downstream
task duration estimates to predict how much of the buffer would be used
up if they complete at that time.
You size the buffers based on the length of the task chain they project.
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 (CCFB). CCPM sets explicit action levels for decisions. The decision
levels are in terms of the buffer size, measured in days:

1. Within the first third of the buffer: Take no action.
2. Penetrate the middle third of the buffer: Assess the problem and
plan for action.
3. Penetrate the final third: Initiate action.
Those measures apply to both the project buffer and the CCFBs.
Figure 4.8 shows an example of using the buffers. The three Xs show
three potential amounts of buffer penetration corresponding to the above
criteria.
Project teams monitor the project buffer and each CCFB at the appropriate time intervals for the project, usually weekly but at least monthly.

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0/3
Project
buffer

119

1/3

2/3

3/3

No action
X
Plan

CCFB-1

X
Act

CCFB-2

Figure 4.8

X

Buffer penetration provides action decisions.

You can update the buffers in a relatively short weekly meeting or use
E-mail status reports. For this tool to be fully useful, the buffer monitoring
time must be at least as frequent as one-third the total buffer time. If the
buffers are negative (i.e., the latest task on the chain is early relative to
schedule date) or less than one-third 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 between one-third and twothirds, 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 by more than two-thirds the buffer size, the project
team should take the planned action. Through this mechanism, buffer
management provides a unique anticipatory project management tool
with clear decision criteria.
Project managers update the buffers as often as they need to simply by
asking each of the performing tasks how many days they estimate to the
completion of their task. They do that without pressure or comment on
the estimate. They expect the estimates to vary from day to day and some
of the tasks to exceed the original duration estimates. As long as the
resources are working on the tasks with 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, as shown in Figure 4.9. The buffer
measure then becomes functionally similar to 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.

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Critical Chain Project Management
Project buffer
1
Act
2/3

x

Plan

x
1/3
0

x

x

x

x

x
x
x

x
Time

Figure 4.9

Plotting trends of buffer penetration may aid analysis.

People have suggested that project managers also monitor buffer
penetration as a percentage of critical chain use. The idea is that the manager should not use up the project buffer too early in the project. Trending
buffer penetration has several advantages over that approach. Perhaps
the most important advantage is that it is simpler to interpret. A second
advantage, especially important if the project task processes are not in
statistical control, is that trending preserves the time history of the data.
That is important information to help improve control of the processes.
Updating the buffers requires that you maintain project status versus
your plan in terms of the tasks completed. It is also a useful direct measure
of project performance.

4.4

Features (more or less) from PMBOK

The unique features of CCPM do not make up a sufficient system to satisfy
the project system requirements identified at the beginning of this chapter. The PMBOK Guide seems to provide all the necessary additional
features to meet the complete system requirements. Following Juran, I
have prepared a feature and requirement correlation matrix to examine
how the CCPM features and selected features and processes from the
PMBOK Guide combine to provide the complete set of identified project
system requirements. (The table is too large for publication in book
format; see [5].) It helped to identify the following set of PMBOK Guide
features as the primary ones necessary to deliver to the requirements
given in Table 4.1. The correlation table also leads to clarification of
the requirements that pertain to each feature and therefore supports
developing the feature.

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The following features, which are (mostly) contained in and explained
in the PMBOK Guide are necessary to satisfy these requirements:
4.4.1

Project charter

The project charter authorizes the initial project team to prepare the
project work plan. It identifies the overall project deliverable, project
stakeholders, overall project responsibilities, and other parameters necessary to create an effective project work plan.
4.4.2

Project work plan

The project work plan identifies the scope, budget, schedule, responsibilities, and resource requirements for the project. It may also specify other
project requirements and plans to achieve them, such as quality, safety,
and regulatory plans. It must contain or reference the operational procedures for the project. Key elements of the project work plan include the
WBS, responsibility assignment, milestone sequencing, work packages,
and the project network.
4.4.2.1

Work breakdown structure

The WBS is the framework to define project scope. It defines project scope
hierarchically, from the complete project level to the work package level.
Work packages complete the hierarchy by specifying the project tasks
necessary to deliver the scope.
4.4.2.2

Responsibility assignment

Responsibility assignment designates the individuals responsible to
accomplish deliverables on the WBS. Responsibility assignment must
occur at the work package level and may be assigned at higher levels.
Responsibility assignment normally confers the authority to perform the
work and accountability for delivering the scope to the budget and schedule for the project deliverable. The person responsible for a work package
does not have to be a task performer, that is, a resource on a task.
4.4.2.3

Milestone sequencing

Milestone sequencing is a tool to go from the hierarchically formatted
WBS to a logical project plan. It provides the major sequence of project