‘PERT’ stands for Program Evaluation and Review Technique. It is also
called "critical path method".
It is an event-oriented planning aid, usually computerised used to
estimate project duration when there is uncertainty in estimates of duration
times for individual activities. It helps the project team to order the
activities that must be completed to implement a decision.
Complex projects require a series of activities, some of which must be
Performed sequentially and others that can be performed in parallel with other
activities. This collection of series and parallel tasks can be modelled as a
network. In 1957 the Critical Path Method (CPM) was developed as a network
model for project management. CPM is a deterministic method
that uses a fixed time estimate for each activity. While CPM is easy to
understand and use, it does not consider the time variations that can have a
great impact on the completion time of a complex project.
The Program
Evaluation and Review Technique (PERT) is a network model that allows for
randomness in activity completion times. PERT was developed in the late 1950's
for the U.S. Navy's Polaris project having thousands of contractors. It has the
potential to reduce both the time and cost required to complete a project.
Steps in the PERT Planning Process
PERT planning involves the following steps:
- 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. It is helpful to list the tasks in a table that in later steps can
be expanded to include information on sequence and duration.
2. Determine Activity Sequence:
This step may be combined with the activity identification step since
the activity sequence is evident for some tasks. Other tasks may require more
analysis to determine the exact order in which they must be performed.
3. Construct the Network Diagram:
Using the activity sequence information, a network diagram can be drawn
showing the sequence of the serial and parallel activities. For the original activity-on-arc
model, the activities are depicted by arrowed lines and milestones are depicted
by circles or "bubbles". If done manually, several drafts may be
required to correctly portray the relationships among activities. Software
packages simplify this step by automatically converting tabular activity
information into a network diagram.
4. Estimate Activity:
Times Weeks are a commonly used unit of time for activity completion,
but any consistent unit of time can be used. A distinguishing feature of PERT
is its ability to deal with uncertainty in activity completion times. For each
activity, the model usually includes three time estimates:
- Optimistic time - generally the shortest time in which the activity can be completed. It is common practice to specify optimistic times to be three standard deviations from the mean so that there is approximately a 1% chance that the activity will be completed within the optimistic time.
- Most likely time - the completion time having the highest probability. Note that this time is different from the expected time.
- Pessimistic time - the longest time that an activity might require. Three standard deviations from the mean is commonly used for the pessimistic time. PERT assumes a beta probability distribution for the time estimates.
5. Determine the Critical:
Path The critical path is determined by adding the times for the
activities in each sequence and determining the longest path in the project.
The critical path determines the total calendar time required for the project.
If activities outside the critical path speed up or slow down (within limits),
the total project time does not change. The amount of time that a non-critical
path activity can be delayed without delaying the project is referred to as
slack time. If the critical path is not immediately obvious, it may be helpful
to determine the following four quantities for each activity:
- ES - Earliest Start time
- EF - Earliest Finish time
- LS - Latest Start time
- LF - Latest Finish time
These times are calculated using the expected time for the relevant
activities. The earliest start and finish times of each activity are determined
by working forward through the network and determining the earliest time at
which an activity can start and finish considering its predecessor
activities.
The latest start and finish times are the latest times that an activity
can start and finish without delaying the project. LS and LF are found by
working backward through the network. The difference in the latest and earliest
finish of each activity is that activity's slack. The critical path then is the
path through the network in which none of the activities have slack.
The variance in the project completion time can be calculated by summing
the variances in the completion times of the activities in the critical path.
Given this variance, one can calculate the probability that the project will be
completed by a certain date assuming a normal probability distribution for the
critical path. The normal distribution assumption holds if the number of
activities in the path is large enough for the central limit theorem to be
applied.
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
Make adjustments in the PERT chart as the project progresses. As the
project unfolds, the estimated times can be replaced with actual times. In
cases where there are delays, additional resources may be needed to stay on
schedule and the PERT chart may be modified to reflect the new situation.
Benefits of PERT are useful because it provides the following
information:
- 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 starts 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.
- Even if the activity times are well-estimated, PERT assumes a beta distribution for these time estimates, but the actual distribution may be different.
- Even if the beta distribution assumption holds, PERT assumes that the probability distribution of the project completion time is the same as that of the critical path. Because other paths can become the critical path if their associated activities are delayed, PERT consistently underestimates the expected project completion time.
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See Also:
- Phases of a Project: Activity levels during various phases of a project
- Planning: Planning in a Project
- Once the project is decided, it has three main goals, describes these in each phase.
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