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382 CHAPTER 9 PROJECT SCHEDULING: PERT/CPM

time. In such situations we need to be able to deal with uncertainty in activity
completion times. We shall show how we do this with the following example.
The Daugherty Porta-Vac Project

Accurate activity time The H.S. Daugherty Company in Ireland has manufactured industrial vacuum
estimates are important cleaning systems for many years. Recently, a member of the company’s new-product
in the development of an research team submitted a report suggesting that the company consider manufactur-
activity schedule. When
activity times are ing a cordless vacuum cleaner. The new product, referred to as Porta-Vac, could
uncertain, the three time contribute to Daugherty’s expansion into the household market. Management hopes
estimates – optimistic, that it can be manufactured at a reasonable cost and that its portability and no-cord
most probable and convenience will make it extremely attractive.
pessimistic – allow the
project manager to take Daugherty’s management wants to study the feasibility of manufacturing the Porta-
uncertainty into Vac product. The feasibility study will recommend the action to be taken. To complete
consideration in this study, information must be obtained from the firm’s research and development
determining the critical (R&D), product testing, manufacturing, cost estimating and market research groups.
path and the activity
schedule. This approach How long will this feasibility study take? In the following discussion, we show how to
was developed by the answer this question and provide an activity schedule for the project.
designers of PERT. Again, the first step in the project scheduling process is to identify all activities
that make up the project and then determine the immediate predecessor(s) for each
activity. Table 9.3 shows these data for the Porta-Vac project.
The Porta-Vac project network is shown in Figures 9.9 and 9.10. Verify that the
network does in fact maintain the immediate predecessor relationships shown in
Table 9.3.
Uncertain Activity Times

However, suppose that the manager in charge of this project has now been informed
that some of the activity times are not guaranteed but are best estimates. After all,
this is quite likely in the real world. Unpredictable factors may affect the completion
of a particular activity: it may take longer than we thought or sometimes we may be
able to complete it earlier than expected. How will this affect the project network,
the current critical path and overall project completion? If we consider Activity A,
for example, which is currently on the critical path, we may now be informed that the
most likely activity time is five weeks but that it could take as long as 12 weeks or, if
everything goes well, we might finish this activity in four weeks. In fact, in many
cases, activity times are uncertain and are best described by a range of possible

Table 9.3 Activity List for the Porta-Vac Project

Immediate Activity
Activity Description Predecessor Time (weeks)

A Develop product design — 5
B Plan market research — 1.5
C Prepare routing (manufacturing engineering) A 3
D Build prototype model A 4
E Prepare marketing brochure A 3
F Prepare cost estimates (industrial engineering) C 2
G Do preliminary product testing D 3
H Complete market survey B, E 3.5
I Prepare pricing and forecast report H 2
J Prepare final report F, G, I 2

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