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Unit 13: PERT, CPM and Time Estimation
The probability associated with Z= +1 is 0.84134. This is a strong probability, and indicates that Notes
the odds are 16 to 3 that the project will be completed by the due date.
If the probability of an event is p, the odds for its occurrence are a to b, where:
a p 0.84134 16
b 1 p 0.15866 3
13.11 Time Estimation
Accurate time estimation is a skill essential for good project management. It is important to get
time estimates right for two main reasons:
1. Time estimates drive the setting of deadlines for delivery and planning of projects, and
hence will impact on other peoples assessment of your reliability and competence as a
project manager.
2. Time estimates often determine the pricing of contracts and hence the profitability of the
contract/project in commercial terms.
Often people underestimate the amount of time needed to implement projects. This is true
particularly when the project manager is not familiar with the task to be carried out. Unexpected
events or unscheduled high priority work may not be taken into account. Project managers also
often simply fail to allow for the full complexity or potential errors and stuff ups, involved with
a project. The 2004-2006 Wembley Stadium project in London is often used as an example,
although there are countless others of less profile.
Time estimates are important as inputs into other techniques used to organise and structure all
projects. Using good time estimation techniques may reduce large projects to a series of smaller
projects.
Step 1 - Understand the Project Outcome
First you need to fully understand what it is you need to achieve. (Refer to my article; Project
Management - Begin with the end in mind). Review the project/task in detail so that there are no
“unknowns.” Some difficult-to-understand, tricky problems that take the greatest amount of
time to solve. The best way to review the job is to just list all component tasks in full detail.
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