Developing the project schedule is the most important task for you to accurately define the duration for each activity.

The 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.

In this blog post, I am going to discuss the three tools to estimate activity duration and how they differ.

Okay, let’s get started.

The three tools to estimate activity duration are as follows:

- Analogous Estimating
- Parametric Estimating
- 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 a completed 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 wall?—You will multiply the time taken to build a ten foot 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 your current 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 PERT (Program Evolution and Review Technique) is the most widely used statistical tool to determine the time duration of an activity.

The PERT 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.

These 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 PERT 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

- It is the fastest technique to calculate the estimate; however, less accurate
- It can be used when limited information about the project is available
- It is also known as top-down estimating

#### Parametric Technique

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

#### Three Points Estimates Technique

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

I hope after reading this post you won’t have any problems in solving the related question in the PMP exam. Let me know if you require any further assistance on this topic.

edward says

Thanks for contribution such helpful information. The information provided is very nice and this information is not available so easily. Therefore I thank the writer for the useful input.

kp says

I second that

Abraham Doyoyo says

The writer, Fahad Usmani, knows his/her job. The information is so accurate and easy to read and understand. Thank you very much. You helped me a lot with your unique information.

Fahad Usmani says

Thanks for your visit and comments Abraham.

Fahad Usmani says

Thank you for such a nice comment.

hisham says

many thanks, but I suggest to add reserve analysis as well.

Fahad Usmani says

This topic is covered under contingency and management reserve analysis blog post.

https://pmstudycircle.com/2012/02/contingency-reserve-vs-management-reserve/

Ram Niwas says

Wondefule explanation. Easily able to understand.

Thanks

Fahad Usmani says

You’re welcome Ram.

Harold Mauricio Camacho hernandez says

Hello Fahad,

There is an additional doubt about estimate durations. I thought that parametric estimating method was defined for the relationship between certain kind of data as, effort – productivity and calendar.

Your post has made me to realize about something, based on the following considerations: If an estimate is not a precise figure but it is a range of time or cost… a parametric estimate based on effort – productivity and calendar is quite precise to be an estimate. then, using those parameters I couldn´t estimate but determine activity duration.

could please clarify this confusion that I have?

Fahad Usmani says

In parametric estimation technique, the estimation is dependent on the data you are using from your past experience. Therefore, you must analyse data for its quality before using it in the parametric estimation technique.

kola sege says

please can you help solve this question…….Tiles (6”X6”) can be installed at the rate of 55 tiles per worker hour. The floor of a large lobby area (80’X50’) is to be covered with these tiles. If three workers are assigned to this task, what duration should be assigned to this activity?

Muhammad Anjum says

Tile size = 6″ x 6″ = 36 SQ.INCH

Tile size = 0.5′ x 0.5′ = 0.25 SQ.FT.

Productivity of 1 worker per hour = 0.25 x 55 = 13.75 SQ.FT.

Productivity of 3 workers per hour = 0.25 x 55 x 3 = 41.25 SQ.FT.

Total lobby area = 80′ x 50′ = 4000 SQ.FT.

Now,

Duration of Activity = Total lobby area / Prod. of 3 workers per hour

Duration of Activity = 4000 / 41.25 = 96.96 hours

If, you devide total no of hours by 8 (hours / day), you can find duration of activity in no. of days = 96.96 / 8 = 12.12 days

Ghada Shkokani says

Thank you very much! It helped a lot! 🙂

Fahad Usmani says

You are welcome Ghada.

Sushil says

Hi Fahad,

PMBOK guide 5 edition also mentions Triangular distribution which also comes under three point distribution. E = (P + O + M) / 3.

Can you discuss the difference between triangular and beta distribution and in which scenarios they would be used.

Do the triangular distribution be used where there is equal chance of best, worst and most likely estimates

Whereas beta distribution would be considered where the chances of most likely estimates are more likely than the other two.

I don’t get the difference between triangular and beta distribution not in terms of formulas but in which cases they are used and how they are different to each other.

Thanks

Sushil

Fahad Usmani says

Okay Sushil, soon I will write a blog post on this topic.

saman says

Hi Farhad, Can you explain to me what is the difference between Project Effort and Duration?

Fahad Usmani says

Hope following blog post will help you understand the difference between project effort and duration:

http://www.pmbypm.com/difference-project-effort-and-duration/

Justin says

If you come to a problem that looks like a PERT / 3 Point question; however, the quest asks which tool do you use, and neither PERT nor 3 Point are there is the answer more likely to be Analogous or Parametric?

Fahad Usmani says

Can not reply based on information given in your comment. Please provide more detail.

Farooq says

Is there any method to calculate duration with respect to man hour…does earned valve is related to schedule?

Fahad Usmani says

As per my understanding earned value parameters should be in dollar terms.