Pert project management:3 time estimates

Pert project management is a technique used for scheduling, controlling the projects.

Pert is defined as Project evaluation and review technique.

Basics of Pert project management technique

PERT uses the project network technique, critical path, activity float.
It assumes that the activities of the project and their network relationships are well defined but if allows for uncertainties in activity durations.
So pert technique is used for scheduling and controlling the projects whose activities possess considerable degrees of certainty in their performance time.
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3 Time estimates for activities in pert project management

The following are the 3-time estimates used for each activity of the pert project management network.
  1. Optimistic time
  2. Most probable time
  3. Pessimistic time

1. Optimistic time:

Optimistic time is the minimum estimate of possible time which activity requires for its completion under ideal conditions.
Type of delay is not included in this time.
it is denoted by ‘to’.

2. Most probable time:

Most probable time is the estimate of time which an activity may take for its
completion under normal conditions.
This estimate is obtained in general by experienced, qualified engineers or foremen.
It includes few normal delays as setbacks which occur in general in executing any project.
It is denoted by the letter ‘tm’.

3. Pessimistic time

Pessimistic time is the maximum time taken by activity if there is a delay at every stage expect natural calamities.
It is the maximum  time for the activity to complete.
It is denoted by the letter ‘tp’.

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The expected time or average time

Expected time is the average time taken by activity if it is repeated a large number of times.
In pert project management  analysis, the expected time of an activity is taken as the weighted average of the three estimate times i.e optimistic time, most probable time, pessimistic time.
While calculating the weighted average time, it is assumed that the optimistic time (to) and pessimistic time (tp) have equal weights whereas most probable activity time ™has
four times weight as compared to the other two.
Mathematically, the expected time may be obtained by the following formula:
te =(( tp + to + 4tm) / 6)

Technical terms used in PERT project management analysis

The following terms used in this pert project management analysis.
  1. Event
  2. Activity
  3. Earliest expected time (Te)
  4. Latest allowable time (Tl)
  5. Slack
  6. Critical path

1. Event

The finish or start or of an activity is called an event.
It neither consumes time nor expenditure.

2. Activity

The actual task performance is called an activity.
It consumes both the time and resources,
It is represented by line or arrow.

3. Earliest expected time (Te)

The earliest expected completion time of the activity is equal to the sum of the expected times of the proceeding activities.

4.Latest allowable time (Tl)

The latest allowable time is the largest possible time as activity can take without delaying the final completion date of the project.

5. Slack

The difference between the earliest expected time and the latest allowable time is called slack.
Slack = Tl-Te

6. Critical path

The path of the network of a project along which there is no slack is called the critical path.

Variance and standard deviation


Variance in statistics is defined as the average squared difference of all numbers from their mean value or average.
The calculation of variation is done by using the formula:
Vt = ((tp-to)/6)2

Standard deviation

Standard deviation in statistics is defined as the square root of variance.
The calculation of standard deviation is done by using the formula:
St = ((tp-to)/6).

What is the difference between CPM and PERT?

The following are the main differences between these networks :


PERT project management  network

CPM project management  network


It is basically analytic
in concept and deals with the problematic network.

It is basically
synthesising in concept and deals with deterministic network


In this network
it uses three-time estimates for activities linking up to two events.

In this network, an only one-time estimate is possible for activities.


It is an event
oriented diagram

It is an activity
oriented diagram


In these times
not related to cost

In these times
are related to cost


It is used for
research development projects and other projects involving uncertainty.

It is used for
repetitive projects such as construction project works, which are based on a
more or less suitable technology.

Related topics:

Critical path method|Project management

About Ashwini Kumar

Ashwini Kumar is a graduate in civil engineering from NIT Warangal. He has 8+ years of experience in the construction field. He is the author, founder of the Easy Home Builds blog

View all posts by Ashwini Kumar →

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