Single Project CCPM

CCPM develops a critical chain, rather than a critical path, as the primary focus of the project. The critical chain includes both logical and resource dependence. CCPM establishes the critical chain after removing resource contentions rather than before considering the resource limitations. The critical chain remains unchanged for the entire duration of the project and is the primary focus of the project manager.

Consider the little project illustrated by Figure 8.1-4. Assuming each task is estimated with each resource working 100 percent of their time on the task, how likely is it that project will finish on time? Most people quickly recognize that it is pretty unlikely because the plan calls for several resources to do two or three tasks at

Figure 8.1-4 An Example Critical Path Project

Figure 8.1-4 An Example Critical Path Project

the same time, which will stretch out those tasks by at least a factor of two or three. Thus, it is unlikely the project would complete as scheduled. This is not news to the world of project management, and numerous approaches to resource leveling can resolve this problem. Figure 8.1-5 illustrates the same project after resource leveling. Note that the project due date moves to the right.

Although the resource-leveling capability exists in most project software, few project managers use it. My informal surveys at the Project Management Institute seminars I give (a large portion of the attendees are certified Project Management Professionals) indicate that only about 5 percent of project managers resource-level. My review of customer project plans indicates more severe planning problems in a large majority of cases, often using scheduling tools to draw Gantt chart pictures with no resource loading or task relationships, much less resource leveling.

Examine Figure 8.1-5 a little closer. Notice what happened to the critical path after resource leveling: every path has a gap in it. The software does not specify the algorithm used to select the particular tasks as critical, and I know that other software (including other versions of the software used) makes different choices. Since all of the paths show float after resource leveling, what should the software do?

Identifying the critical chain resolves this conflict. The critical chain is the longest path through the network after resource level-

Figure 8.1-5 The Resource-Leveled Critical Path Project


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ing. The critical chain has no float or slack when identified. It usually differs from the critical path, as it can jump the task logic network. Figure 8.1-6 illustrates the critical chain for the Figure 8.1-5 network, comprising WBS 1.1, 1.2, 2.2, 3.2, 3.3, 4.1, and 4.2. Later steps in creating the complete critical chain network may introduce apparent float or slack into the network.

Figure 8.1-6 also illustrates the reduction of activity duration, and additions of buffers. Four feeding buffers are inserted as WBS FB19, FB22, FB18, FB20. The feeding buffers help ensure that both the inputs and the resources are available to start critical chain tasks. The critical chain scheduled project duration is about the same as the resource-leveled critical path, including the project buffer. You should expect completion before the end of the project buffer, and half the time before the start of the project buffer.

CCPM uses mean (roughly 50 percent) probability activity duration estimates and an aggregated project buffer to deliver the project on time. This significantly reduces the scheduled project lead time and significantly increases the probability of completing the project.

Figure 8.1-6 Identifying the Constraint to a Single Project: The Critical Chain and Adding Buffers


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Note: The critical chain appears as the light gray bars in this graphic. They would typically be red in a color graphic.

Note: The critical chain appears as the light gray bars in this graphic. They would typically be red in a color graphic.

CCPM solves the merging activity problem (the fact that an activity with multiple predecessors with finish-to-start relationships cannot start until the last predecessor completes) and the early-start, late-finish dilemma (Which way to schedule?) by the use of feeding buffers. Inserting FBs where each activity chain feeds the critical chain (including the entry to the project buffer) helps to immunize the critical chain from delay in these feeding paths. Late-starting the feeding chains against these buffers, as allowed by resource leveling, resolves the early/late start question.

Consider an activity on the critical chain that requires inputs from three tasks; one is the immediate predecessor on the critical chain and the other two on parallel network paths. If the tasks are estimated with a fifty/fifty chance of completing each task within the duration estimate, the chance of having all three is only one-eighth (probabilities multiply). The latest of the three tasks will determine the start time of the common successor task. The feeding buffers add extra time to the noncritical chain paths, moving the predecessors earlier in time so that the chance of having each of those feeder chain inputs is very high. This increases the chance of having all three predecessors, and thus the start of the successor task, back up to near fifty/fifty.

The feeding buffers (combined with the activity-dependent schedule created with establishing the critical chain) allow starting activities as late as possible, while protecting the overall project, because the feeding buffers add enough time to ensure the feeding chains are complete when needed (to a high probability). The scheduled start of the feeding chains will be later than early-start times, giving the project the maximum focus and cash flow advantages from starting later. Compare the start times of tasks 2.1 and 3.1 in Figure 8.1-6 to their start times in Figure 8.1-4 to see this effect.

CCPM uses buffer management during project execution to answer two primary questions:

For project and task managers: "Which task do I work on next?"

For the project manager, "When do I take actions to accelerate the project?"

Tracking TOC projects requires identifying when tasks start and finish and obtaining estimates on the remaining duration for tasks in work. The reason for using remaining duration rather than estimates of completion is that humans tend to overestimate the percentage complete. When called on to look forward and consider the work remaining to complete a task, more accurate estimates are obtained. Remaining duration is also the actual number needed to project completion, and estimating it directly avoids the assumptions necessary to convert a percentage complete estimate to a remaining duration estimate.

CCPM project tracking uses the estimates of remaining duration for incomplete tasks to calculate the impact of the task status, including the absorption of variation by feeding buffers, to determine how much of the project buffer has been used. Priority is placed on the tasks that cause the greatest amount of project buffer penetration. Using task priority in this way enables resources to focus on one project task at a time, thereby completing it in the minimum possible time. Tasks do not have due dates. This helps avoid having Parkinson's Law (task durations extend to use available time) or Student Syndrome (waiting to start a task until the due date is urgent) cause late task delivery. The ability to update remaining duration after tasks start also encourages using mean task duration estimates.

The mechanism to complete projects as soon as possible answers two different questions. The answer to the first question, "Which project task should I work on next?" addresses the task and resource manager's need to enable relay-racer-like task performance, avoiding bad multitasking. The answer to the second question, "When should we take action to recover schedule?" helps the project team decide when to take action to recover buffer that is being used up at too high a rate.

Figure 8.1-7 illustrates a task manager view into a CCPM project that is underway. The tasks are color coded in the task number box on the left (not visible in the graphic) to highlight the priority of the task. Red tasks (in this graphic, the first item) get the highest priority, as they are on a path that is causing significant project

Figure 8.1-7 Critical Chain Software Updates Tasks Using Remaining Duration, Prioritizing Tasks to Be Worked On

Figure 8.1-7 Critical Chain Software Updates Tasks Using Remaining Duration, Prioritizing Tasks to Be Worked On

Source: Used by permission from Realization, Inc.

buffer use. The Concerto software used to generate this screen shot is the only multiproject CCPM software that I know of that directly provides the task level priority for the multiproject environment.

The amount of project buffer penetration also answers the second question by providing the signal to take proactive action to recover buffer (see Figure 8.1-8). If the buffer is in the yellow region (the middle in this figure), plans are developed to recover buffer. If the buffer penetration moves into the red region (the upper portion in the figure), the buffer recovery actions are implemented. This approach causes the project team to focus on the tasks delaying the project versus those that might earn the most value. Figure 8.1-8 also shows the trend of buffer penetration, enabling anticipatory action and easy determination of the efficacy of buffer recovery action.

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