Developing the project schedule is the most important task for you to accurately define the duration for each activity. Soundness of your schedule heavily depends on the accuracy of the duration of all project activities.

There are three basic techniques most widely used by the project professionals worldwide to estimate activity duration and build the schedule. Those techniques are as follows:

1. Analogous Estimating
2. Parametric Estimating
3. Three Point Estimates

Analogous Estimating 

The Analogous Estimates process uses analogies from the earlier similar projects.  For example, let’s say you have to estimate the duration of a school building project with the help of the analogous estimating. In this case, you will look into your organizational process assets (OPA) for any earlier school building project completed by your organization and pick the best project, which looks similar to your project.

Once you find the old similar project, you will compare it with your project and use your expert judgment to find the approximate time duration for your current project.

With the analogous method of estimating, you can estimate the duration of a project very quickly. This process is not very accurate; however, it is very useful when there is very little information available for the project and you have to estimate the duration as quickly as possible.

The accuracy of analogous estimation depends on the degree of similarities between your project and the project you are comparing it with.

Parametric Estimating

The Parametric Estimation is somewhat similar to the analogous estimation because it also uses the past records to compute the duration estimate. However, this approach is different from the analogous estimation.

The parametric estimation uses historical data and other parameters to calculate the estimate.  For example, if constructing a ten foot wall took one day, then how long will it take to build a hundred foot of wall? —You will multiply the time taken to build ten foot of wall in the old project by ten.

Or, if one room is painted in three days in an earlier project, how much time it will take to paint twenty rooms in this project? —You will multiply the time taken to paint one room in the old project by twenty to get the required number of days for your project.

The accuracy of the duration estimate obtained from this technique is better than that from the analogous estimation.

Three Points Estimates

Under the three-point estimates techniques, the PEART (Program Evolution and Review Technique) is the most widely used statistical tool to determine the time duration of an activity.

The PEART is a weighted average technique to determine the approximate duration of an activity. It uses three time estimates to determine an approximate average duration of an activity.
Three time estimates are as follows:

• Most Likely (T_m): This is the time taken by an activity to finish it in most cases
• Optimistic (T_o): This is the time taken by an activity to finish it in the most favorable case
• Pessimistic (T_p): This is the time taken by an activity to finish it in the worst case scenario

Once you get these three estimates, you can calculate the PEART Time Estimate by using the below given formula:

Te = (T_o + 4T_m + T_p)/6

The duration estimate obtained by this method is more accurate than the rest. Using three points estimate reduces the chances of risks, bias judgment, and uncertainty.

Key Points:

Analogous Technique

• is the fastest technique to calculate the estimate; however, less accurate
• can be used when limited information about the project is available
• is also known as Top-Down Estimating

Parametric Technique

• uses statistical relationships between historical data and variables
• is more accurate than the analogous technique

Three Points Estimates Technique

• uses three estimates (most likely, optimistic, and pessimistic) to calculate the average value of activity duration
• reduces the bias, risks and uncertainties from the duration calculation
• more accurate than the rest

Here this blog post finishes. I hope after reading this post you wouldn’t have problem in cracking the relaed question in the PMP Test. Let me know if you require any further assistance on this topic.

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