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372 CHAPTER 9 PROJECT SCHEDULING: PERT/CPM

PERT (Navy) and CPM PERT was developed in the late 1950s specifically for the Polaris missile project
(Du Pont and Remington in the USA. Many activities associatedwiththisproject hadnever been
Rand) differ because
they were developed by attempted previously, so PERT was developed to handle uncertain activity times.
different people working CPM was developed primarily for industrial projects for which activity times were
on different projects. known. CPM offered the option of reducing activity times by adding more work-
Today, the best aspects ers and/or resources, usually at an increased cost. So, a distinguishing feature of
of each have been
combined to provide a CPM was that it identified trade-offs between time and cost for various project
valuable project activities.
scheduling technique. Today’s computerized versions of PERT and CPM combine the best features of
both approaches. So, the distinction between the two techniques is no longer
necessary. As a result, we refer to the project scheduling procedures covered in this
chapter as PERT/CPM.

9.1 Project Scheduling With Known Activity Times

The effort that goes into The owner of the Souk al Bustan (SaB) Shopping Centre in Dubai is planning to
identifying activities, modernize and expand the current 32-business shopping centre complex. The proj-
determining ect is expected to provide room for eight to ten new retail businesses. Financing has
interrelationships among
activities and estimating been arranged through a private investor. All that remains is for the owner of the
activity times is crucial to shopping centre to plan, schedule and complete the expansion project. Let us show
the success of PERT/ how PERT/CPM can help.
CPM. A significant The first step in the PERT/CPM scheduling process is to develop a list of the
amount of time may be
needed to complete this activities that make up the project. Table 9.1 shows the list of activities for the
initial phase of the Shopping Centre expansion project. Nine activities are described and denoted
project scheduling A through I for later reference. Table 9.1 also shows the immediate predecessor(s)
process.
and the activity time (in weeks) for each activity. For a given activity, the immediate
predecessor column identifies the activities that must be completed immediately
Immediate predecessor prior to the start of that activity. Activities A and B do not have immediate prede-
information determines
whether activities can be cessors and can be started as soon as the project begins; so, a dash is written in
completed in parallel the immediate predecessor column for these activities. The other entries in the
(worked on immediate predecessor column show that activities C, D and E cannot be started
simultaneously) or in until activity A has been completed; activity F cannot be started until activity E
series (one completed
before another begins). has been completed; activity G cannot be started until both activities D and F have
Generally, the more been completed; activity H cannot be started until both activities B and C have been
series relationships completed; and, finally, activity I cannot be started until both activities G and H have
present in a project, the been completed. The project is finished when activity I is completed.
more time will be
required to complete the The last column in Table 9.1 shows the number of weeks required to complete
project. each activity. For example, activity A takes five weeks, activity B takes six weeks and
so on. The sum of activity times is 51. As a result, you may think that the total time
required to complete the project is 51 weeks. However, as we show, two or more
A project network is activities may often be scheduled concurrently, so shortening the completion time
extremely helpful in
visualizing the for the project. Ultimately, PERT/CPM will provide a detailed activity schedule for
interrelationships among completing the project in the shortest time possible.
the activities. No rules Using the immediate predecessor information in Table 9.1, we can construct a
guide the conversion of a graphical representation of the project, or the project network. Figure 9.1 depicts
list of activities and
immediate predecessor the project network for the Shopping Centre. The activities correspond to the nodes
information into a project of the network (drawn as rectangles) and the arrows show the precedence relation-
network. The process of ships among the activities. In addition, nodes have been added to the network to
constructing a project denote the start and the finish of the project. A project network will help a manager
network generally
improves with practise visualize the activity relationships and provide a basis for carrying out the PERT/
and experience. CPM calculations.

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