Differentiate between Optimistic Time Estimate and Pessimistic Time Estimate ?

Differentiate between Optimistic Time Estimate and Pessimistic Time Estimate ?

Time Estimate in PERT (Project Evaluation and Review Technique)

Before any activity begins related to the work of a project, every project requires an advanced, accurate time estimate. Without an accurate estimate, no project can be completed within the budget and the target completion date.

There are three estimation times involved in PERT

  1. Optimistic Time Estimate (OPT)
  2. Most Likely Time Estimate (MLT)
  3. Pessimistic Time Estimate (PT)

In PERT, these three estimate times are derived for each activity. This way, a range of time is given for each activity with the most probable value.


Following are further details on each estimate:

1. Optimistic Time Estimate (OPT)

  • The estimate of minimum possible time in which an activity can complete under ideal condition is called optimistic time estimate.
  • This is shortest possible time.
  • So fastest time to complete activity.
  • In this we assume that all the resources are available.
  • It does not delay at any stage.
  • It is generally denoted by ‘to


2. Most Likely Time Estimate (MLT)

  •  The most probable estimate of time in which an activity successfully completed under normal conditions is called Most likely time estimate.
  • This time is basis on the previous experience of qualified engineers or foreman.
  • It allow few delay which is generally happen in execution of any project
  • It is generally denoted by ‘tm


3. Pessimistic Time Estimate

  • The maximum time taken by an activity with delay is called Pessimistic time estimate.
  • If there is any delay at every stage is included.
  • It excludes the natural calamities
  • It is the longest time taken
  • It is denoted by ‘tp


Expected Time or Average Time

Average Time taken by an activity if it is repeated many times is called expected time.

It is denoted by te

te = (to + 4 tm + tp)/6


The possible variance (V) of the estimate is calculated as below:

V = (tp - to)^2 / 6^2


The most pessimistic (tp) case – when everything goes wrong.

The most optimistic (to) case – where everything goes right.

The most likely (tm) case – given normal problems and opportunities.

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