Page 175 - DCAP304_DCAP515_SOFTWARE_PROJECT_MANAGEMENT

Unit 9: CPM /PERT

new chemical plant. In March 1959, the method was applied to a maintenance shutdown at Notes
the Du Pont works in Louisville, Kentucky. Unproductive time was reduced from 125 to 93
hours.

Self Assessment

Fill in the blanks:
1. Compute the longest time path through the network. This is called the …………………...

2. In certain cases, the activities are…………………., because they are independent of each
other and can start simultaneously.
3. Non-critical activities can be replanned, rescheduled and resources for them can be
………………………… flexibly, without affecting the whole project.
4. Use the Network to help plan, schedule, ……………………. and control the project.

9.2 CCPM vs Traditional Project Management

CCPM (Critical Chain Project Management) puts more emphasis on the resources required to
execute project tasks in contrast to the more traditional Critical Path and PERT methods, which
emphasize task order and rigid scheduling.
Secondly, using the Critical Chain Method, projects can be completed more quickly and with
greater scheduling reliability. The difference between traditional and Critical Chain scheduling
is in how uncertainty is managed. In traditional project scheduling, uncertainty is managed by
padding task durations, starting work as early as possible, multi-tasking, and focusing on meeting
commitment dates. The following points illustrate some of the problems associated with
traditional project scheduling:
 Padding task durations (providing worst-case estimates) is done to ensure a high probability
of task completion. The knowledge that there is so much safety time built into tasks
results in various time wasting practices, e.g., waiting until the last moment to complete
a task. As a result, all the safety time can be wasted at the start of the task so that, if
problems are encountered, the task overruns.

 Starting work as early as possible, even when not scheduled, is a response to worst-case
estimates. When workers give worst-case estimates, they don’t expect to stay busy with just
one task – so they multi-task, working on several tasks at once by switching between them.
The result is that everything takes a long time to complete and very little completes early.
 With the focus on meeting commitment dates (start and finish), output from a task
completed early will rarely be accepted early by the next person needing this output. So,
any effort spent in finishing early will be wasted.
Given the above issues, the most projects are always late.

In critical chain scheduling, uncertainty is primarily managed by:
(a) using average task duration estimates
(b) scheduling backwards from the date a project is needed (to ensure work that needs to be
done is done, and it is done only when needed)
(c) placing aggregate buffers in the project plan to protect the entire project and the key tasks
and using buffer management to control the plan. The key tasks are those on which the
ultimate duration of the project depends, also known as the Critical Chain.

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