Estimate activity duration is an important project management process that helps you develop the project schedule. The accuracy of it depends on this process.

You use four tools to find the duration of your project or activities. The accuracy depends on when you do the estimation and the availability of the information.

As a project manager, you will use these techniques to provide duration estimates to your superiors. Understanding these terms is necessary.

Each activity has three attributes: resources required, the effort required and duration.

These attributes depend on each other. The more resources, the faster an activity can be completed. Moreover, an expert team member will take less time to complete a task than a novice team member. Therefore, first, apply the resources and effort. Then you will calculate the activity duration.

### Estimate Activity Duration Tools

The following are four tools to estimate activity duration:

- Analogous Estimating
- Parametric Estimating
- Three Points Estimating
- Bottom-up Estimating

#### Analogous Estimating

You can use this technique on a project level or an activity level for a quick result.

Analogous estimates use analogies from past similar projects. For example, assume you have to estimate the duration of a project, so you will look into your organizational process assets (OPA) for a similar completed project. You will then pick the project that resembles yours the most. Using your expert judgment, you will choose the duration estimate for your current project.

This technique is useful when you have little information about your project, and management needs quick answers. The accuracy of this estimation depends on the similarities between your project and the project you are comparing it to.

This technique is also known as “Top-down Estimation.”

**Key Points:**

- It is the fastest technique.
- This technique does not provide an accurate estimation.
- You can use this with a few project details.

#### Parametric Estimating

This technique uses historical records to compute the duration estimate. Parametric estimation seems similar to analogous estimation but it has a very different approach.

This uses statistical, historical data to find 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? Multiply the time taken to build a ten-foot wall by ten.

Or, if one room took three days to paint in an earlier project. How much time it will take to paint twenty rooms on your current project? You will multiply the time taken to paint one room on the old project by twenty to get the duration for your new project.

This technique is more accurate than the analogous estimation.

**Key Points: **

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

#### Three Points Estimates

You can reduce bias using three-point estimates.

Triangular and beta distribution formulas are the most popular three-point estimates.

In three-point estimates, we use three parameters:

- Most Likely (Tm): This is the activity duration for most cases.
- Optimistic (To): This is the activity duration in the most favorable case.
- Pessimistic (Tp): This is the activity duration in the worst-case scenario.

The triangular estimate formula is:

Te = (To + Tm + Tp) / 3

PERT (Program Evaluation and Review Technique) is the most widely used three-point estimate technique.

It is a weighted average. The PERT estimate formula is:

Te = (To + 4Tm + Tp) / 6

Using a three-point estimate reduces the chance of risks, biased judgment, and uncertainty.

Key Points:

- It uses three estimates to calculate the average value of an activity’s duration.
- It reduces bias, risks, and uncertainties from the duration calculation.
- It is more accurate than analogous and parametric estimation.

### Bottom-up Estimating

Here, you will calculate the activities’ duration with the highest level of precision and add them together to get your project duration.

This is the most accurate, most time-consuming, and the most costly technique. It is also known as a “definitive estimate.”

**Key Points:**

- This is the most accurate technique and provides reliable results.
- You can use this technique when you have many details.
- This is a costly and time-consuming technique.

### Summary

Knowing accurate activity durations helps you create a realistic schedule and helps you complete the project with minimal obstruction. Activity duration is an iterative process. You can repeat this process throughout the project life cycle as needed. It helps you establish the schedule baseline of the project.

This is an important process for your project’s success. Any deviation from the schedule baseline can affect your cost baseline. This is bad for your project and can damage the credibility of your organization.

This is an important topic from a PMP exam point of view. You will a few questions from this topic in your PMP exam.

Thanks a lot very brief and informative.

You are welcome Saima.

Please, i need your view on this as well. Activity resource estimating.

What you want me to explain you?

Please, can help me solve this question. what are the tools and techniques for activity duration estimating.

I cannot share it on the blog due to copyright issues. It is better if you look into the PMBOK Guide.

hi sir, I need a schedule of a residential building if you manage, it will be very helpful.

please help…thanks

Sorry Hamza, I don’t have it.

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

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

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?

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

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

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

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

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

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

Thank you very much! It helped a lot! 🙂

You are welcome Ghada.

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?

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

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?

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.

Wondefule explanation. Easily able to understand.

Thanks

You’re welcome Ram.

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

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

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

Thank you for such a nice comment.

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.

I second that

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.

Thanks for your visit and comments Abraham.