Lecture_26_CPM / PERTNetwork Diagram

26.1 Introduction to CPM / PERT Techniques
CPM (Critical Path Method) was developed by Walker to solve project scheduling problems.
PERT (Project Evaluation and Review Technique) was developed by team of engineers
working on the polar’s missile programme of US navy.
The methods are essentially network-oriented techniques using the same principle. PERT and
CPM are basically time-oriented methods in the sense that they both lead to determination of a
time schedule for the project. The significant difference between two approaches is that the time
estimates for the different activities in CPM were assumed to be deterministic while in PERT
these are described probabilistically. These techniques are referred as project scheduling
techniques.
26.2 Applications of CPM / PERT
These methods have been applied to a wide variety of problems in industries and have found
acceptance even in government organizations. These include
? Construction of a dam or a canal system in a region
? Construction of a building or highway
? Maintenance or overhaul of airplanes or oil refinery
? Space flight
? Cost control of a project using PERT / COST
? Designing a prototype of a machine
? Development of supersonic planes
26.3 Basic Steps in PERT / CPM
Project scheduling by PERT / CPM consists of four main steps
1. Planning
? The planning phase is started by splitting the total project in to small projects. These
smaller projects in turn are divided into activities and are analyzed by the department or
section.
? The relationship of each activity with respect to other activities are defined and
established and the corresponding responsibilities and the authority are also stated.
Lecture 26
CPM / PERT
Network Diagram
1
? Thus the possibility of overlooking any task necessary for the completion of the project is
reduced substantially.
2. Scheduling
? The ultimate objective of the scheduling phase is to prepare a time chart showing the start
and finish times for each activity as well as its relationship to other activities of the
project.
? Moreover the schedule must pinpoint the critical path activities which require special
attention if the project is to be completed in time.
? For non-critical activities, the schedule must show the amount of slack or float times
which can be used advantageously when such activities are delayed or when limited
resources are to be utilized effectively.
3. Allocation of resources
? Allocation of resources is performed to achieve the desired objective. A resource is a
physical variable such as labour, finance, equipment and space which will impose a
limitation on time for the project.
? When resources are limited and conflicting, demands are made for the same type of
resources a systematic method for allocation of resources become essential.
? Resource allocation usually incurs a compromise and the choice of this compromise
depends on the judgment of managers.
4. Controlling
? The final phase in project management is controlling. Critical path methods facilitate the
application of the principle of management by expectation to identify areas that are
critical to the completion of the project.
? By having progress reports from time to time and updating the network continuously, a
better financial as well as technical control over the project is exercised.
? Arrow diagrams and time charts are used for making periodic progress reports. If
required, a new course of action is determined for the remaining portion of the project.
26.4 Network Diagram Representation
In a network representation of a project certain definitions are used
1. Activity
Any individual operation which utilizes resources and has an end and a beginning is called
activity. An arrow is commonly used to represent an activity with its head indicating the
direction of progress in the project. These are classified into four categories
1. Predecessor activity – Activities that must be completed immediately prior to the start of
another activity are called predecessor activities.
2. Successor activity – Activities that cannot be started until one or more of other activities
are completed but immediately succeed them are called successor activities.
3. Concurrent activity – Activities which can be accomplished concurrently are known as
concurrent activities. It may be noted that an activity can be a predecessor or a successor
to an event or it may be concurrent with one or more of other activities.