PERT Weighted Average
(expected time) (mean)=(O+4M+P)/6
Sandard Deviation (SIGMA)=(PO)/6
Variance =(Standard Deviation)2
Lowest= mean(Sigma*6)
Highest=mean+(sigma*6)
1 Sigma 68.3% 2 sigma
95.5% 3 sigma 99.7 %
6 sigma 99.99%
PERT
CPM
is a method that uses a fixed time
estimate for each activity based on historical , expert references. CPM does not consider the time variations that
may
have a great impact on the completion time of a complex project.
The
Program Evaluation and Review Technique
(PERT)
.emphasizes
future probabilities in time estimates (optimistic, pessimistic, and
expected time)
The Network Diagram
PERT originally was an
activity on arc network, in which the activities are represented on the lines and milestones on the nodes. Over time, people began to use PERT as an
activity on node network.
YearsLater
CPM and PERT were developed, the two terms are used interchangeably as each
technique has borrowed from one other. Currently, two network techniques are
used extensively in project management scheduling networks: ActivityonArc
(AOA) (is
also known as ActivityonArrow (because an arc is sometimes called an
arrow). When you use AOA, you use arcs or arrows, called activities, to
connect events. Consequently, AOA is eventoriented
uses DUMMY
Activity)
and
ActivityonNode (AON) Instead of
circles and labels on arcs, AON uses a userfriendly flowcharting approach.)
It is the most popular
project management network scheduling technique for complex, large projects. The
name (ActivityonNode, AON) is not widely used. Most calls it a PERT
chart.
Key
Network Scheduling Principles
Regardless what network scheduling technique you use, understand and
follow these key principles:
Earliest start: The earliest an activity can begin based on its relationship to its predecessor's duration.
Latest start: The latest an activity can begin and still satisfy requirements set forth by successor activities.
Earliest finish: Based on the relationship with a predecessor, the earliest an activity can be completed.
Latest finish: Based on the relationship with a successor, the latest an activity can be completed.
Float time: The amount of free time (slack time) an activity has without impacting due date of the project.
Forward pass: A technique that works forwards, starting with the first node in the network,
to determine earliest start and finish times.
Backward pass: A technique that works backwards, starting with the last node in the network,
to determine latest start and finish times.
Steps in the PERT Planning Process
Identify the specific activities and milestones.
Determine the proper sequence of the activities.
Construct a network diagram.
Estimate the time required for each activity.
Determine the
critical path.
Update the PERT chart as the project progresses.
1. Identify Activities and Milestones
The activities are the tasks required to
complete the project.
The milestones are the events marking the beginning and
end of one or more activities.
2. Determine Activity Sequence
3. Construct the Network Diagram
4. Estimate Activity Times

Optimistic time
 generally the shortest time in which the activity can be completed.

Most likely time  the completion time having the highest probability.

Pessimistic time
 the longest time that an activity might require.
5. Determine the Critical Path
Since the critical path determines the completion date of the project, the project can be accelerated by adding the resources required to decrease the time for the activities in the critical path. Such a shortening of the project sometimes is referred to as
project crashing.
6. Update as Project Progresses
Benefits of PERT

Expected project completion time.

Probability of completion before a specified date.

The critical path activities that directly impact the completion time.

The activities that have slack time and that can lend resources to critical path activities.

Activity start and end dates.
Limitations
The following are some of PERT's weaknesses:

The activity time estimates are somewhat subjective and depend on judgement. In cases where there is little experience in performing an activity, the numbers may be only a guess. In other cases, if the person or group performing the activity estimates the time there may be bias in the estimate.

the actual distribution may be different.

other paths can become the critical path
if their associated activities are delayed, PERT consistently underestimates the
expected project completion time.
To overcome , Monte Carlo simulations can be
performed to eliminate this optimistic bias in the expected project
completion time.
The second
advantage of PERT is that one can determine the probability of meeting
specific deadlines by development of alternative plans. If the
decision maker is statistically sophisticated, he can examine the
standard deviation and the probability of accomplished data.
So, standard deviation gives an idea of the probability of whether a
task will happen. .