Page 236 - DMGT521_PROJECT_MANAGEMENT

Unit 13: PERT, CPM and Time Estimation

Notes
Table 13.3

Duration Earliest Earliest Latest
Event Latest Start Total Float
(Weeks) Start Finish Finish
1-2 4 0 4 0 4 0
2-3 0 4 4 4 4 0
3-4 7 4 11 4 11 0

1-3 3 0 3 1 4 1
2-4 6 4 10 5 11 1

The Earliest Start is the value in the rectangle near the tail of each activity.
1. The Earliest Finish is = Earliest Start + Duration
2. The Latest Finish is the value in the diamond at the head of each activity
3. The Latest Start is = Latest Finish - Duration

There are two important types of Float or Slack. These are Total Float and Free Float.
TOTAL FLOAT is the spare time available when all preceding activities occur at the earliest
possible times and all succeeding activities occur at the latest possible times.

Total Float = Latest Start - Earliest Start
Activities with zero Total float are on the Critical Path
FREE FLOAT is the spare time available when all preceding activities occur at the earliest possible
times and all succeeding activities occur at the earliest possible times.
When an activity has zero Total float, Free float will also be zero.
There are various other types of float (Independent, Early Free, Early Interfering, Late Free, Late
Interfering), and float can also be negative. We shall not go into these situations at present for
the sake of simplicity and be concerned only with Total Float for the time being.
Having computed the various parameters of each activity, we are now ready to go into the
scheduling phase, using a type of bar chart known as the Gantt Chart.
There are various other types of float (Independent, Early Free, Early Interfering, Late Free, Late
Interfering), and float can also be negative. We shall not go into these situations at present for
the sake of simplicity and be concerned only with Total Float for the time being. Having
computed the various parameters of each activity, we are now ready to go into the scheduling
phase, using a type of bar chart known as the Gantt Chart.

Did u know? TOTAL FLOAT is the spare time available when all preceding activities occur
at the earliest possible times and all succeeding activities occur at the latest possible times.

13.10 The PERT (Probabilistic) Approach

So far we have talked about projects, where there is high certainty about the outcomes of
activities. In other words, the cause-effect logic is well known. This is particularly the case in
Engineering projects.

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