Page 234 - DMGT521_PROJECT_MANAGEMENT

Unit 13: PERT, CPM and Time Estimation

Non-critical activities can be replanned, rescheduled and resources for them can be reallocated Notes
flexibly, without affecting the whole project.
Five useful questions to ask when preparing an activity network are:

1. Is this a Start Activity?
2. Is this a Finish Activity?
3. What Activity Precedes this?
4. What Activity Follows this?

5. What Activity is Concurrent with this?
Some activities are serially linked. The second activity can begin only after the first activity is
completed. In certain cases, the activities are concurrent, because they are independent of each
other and can start simultaneously. This is especially the case in organisations which have
supervisory resources so that work can be delegated to various departments which will be
responsible for the activities and their completion as planned. When work is delegated like this,
the need for constant feedback and coordination becomes an important senior management
preoccupation.

Drawing the PERT/CPM Network

Each activity (or sub-project) in a PERT/CPM Network is represented by an arrow symbol. Each
activity is preceded and succeeded by an event, represented as a circle and numbered.

Figure 13.12: PERT/CPM Network

At Event 3, we have to evaluate two predecessor activities - Activity 1-3 and Activity 2-3, both of
which are predecessor activities. Activity 1-3 gives us an Earliest Start of 3 weeks at Event 3.
However, Activity 2-3 also has to be completed before Event 3 can begin. Along this route, the
Earliest Start would be 4+0=4. The rule is to take the longer (bigger) of the two Earliest Starts. So
the Earliest Start at event 3 is 4.
Similarly, at Event 4, we find we have to evaluate two predecessor activities - Activity 2-4 and
Activity 3-4. Along Activity 2-4, the Earliest Start at Event 4 would be 10 wks, but along Activity
3-4, the Earliest Start at Event 4 would be 11 wks. Since 11 wks is larger than 10 wks, we select it
as the Earliest Start at Event 4. We have now found the longest path through the network. It will
take 11 weeks along activities 1-2, 2-3 and 3-4. This is the Critical Path.

LOVELY PROFESSIONAL UNIVERSITY 229