estimating activity duration

Estimating activity duration is an important project management process that helps you develop the project schedule.

Several tools help you determine activity duration. The choice of tool depends on the accuracy you are looking for and the available information. 

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

To calculate activity duration estimates, you need to understand these attributes.

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, identify the resources and effort, then estimate the activity duration.

Estimating Activity Duration

You have four techniques to calculate activity duration in project management:

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

Below is a description of each technique and when it should be used to find the activity duration.

Analogous Estimating

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

Analogous estimates use comparisons from similar past projects. 

For example:

You need to estimate the duration of a project, so you will look into your organizational process assets (OPA) for a similar completed project, finding the one that resembles yours the most. Using your expert judgment, you will calculate the duration estimate for your current project.

This technique is useful when you have scant 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.

When you should use this technique to estimate activity duration.

You can use this technique to find activity duration when you need a quick result and accuracy is not required. 

This technique is useful when you have limited activity details.

Parametric Estimating

The parametric estimating technique uses historical records to compute the duration of an activity. Parametric estimation seems similar to analogous estimation, but it has a very different approach.

This uses historical statistical data to find the duration. 

For example, if constructing a ten-foot wall took one day, 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 will it 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 analogous estimation.

When you should use this technique to estimate activity duration.

This technique provides a more accurate duration estimate than the analogous technique. You use this technique when you have a recent similar project.

Three-Point Estimates

You can reduce bias using the three-point estimates technique, which uses a mathematical formula to calculate activity durations. 

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.

When you should use this technique to estimate activity duration.

This technique provides a better activity duration estimate than analogous or parametric. It reduces bias, risks, and uncertainties from the duration calculation.

You use this technique when you have three duration estimates for an activity.

Bottom-up Estimating

The bottom-up estimating technique calculates the activities’ duration with the highest level of precision and adds them together to get your project duration.

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

When you should use this technique to estimate activity duration.

You can use this activity duration technique when you have a full grasp of all activity details available. This is the most accurate technique and provides reliable results.

However, this is the most time-consuming technique and requires extensive resources.

Summary

Estimate activity duration is an important process, used after you identify activities and find the resources required to complete the task. Knowing accurate activity durations helps you create a realistic schedule and avoid surprises. Activity duration is an iterative process, meaning you will repeat this process throughout the project life cycle as needed.

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

Please share your experience with activity duration estimates on your project through the comments section.