Planning

Duration estimates worksheet

A duration estimating worksheet can help to develop duration estimates when quantitative methods are used. Quantitative methods include:

  • Parametric estimates
  • Analogous estimates
  • Three-point estimates

Parametric estimates are derived by determining the effort hours needed to complete the work. The effort hours are then calculated by:

  • Dividing the estimated hours by resource quantity (i.e., number of people assigned to the task)
  • Dividing the estimated hours by the percent of time the resource(s) are available (i.e., 100 percent of the time, 75 percent of the time, or 50 percent of the time)
  • Multiplying the estimated hours by a performance Experts in a field generally complete work faster than people with an average skill level or novices. Therefore, a factor to account for the productivity is developed.

Duration estimates can be made even more accurate by considering that most people are productive on project work only about 75 percent of the time.

Analogous estimates are derived by comparing current work to previous similar work. The size of the previous work and the duration is compared to the expected size of the current work compared to the previ- ous work. Then the ratio of the size of the current work is multiplied by the previous duration to determine an estimate. Various factors, such as complexity, can be factored in to make the estimate more accurate. This type of estimate is generally used to get a high-level estimate when detailed information is not available.

A three-point estimate can be used to account for uncertainty in the duration estimate. Stakeholders provide estimates for optimistic, most likely, and pessimistic scenarios. These estimates are put into an equation to determine an expected duration. The needs of the project determine the appropriate equation, but a common equation is the beta distribution:

 

Estimated duration =  Optimistic duration+ 4 most likely duration+ pessimistic duration
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In formulas, duration is often represented by “t” for “time.”

The duration estimating worksheet can receive information from:

  • Assumption log
  • Scope baseline
  • Activity list
  • Activity attributes
  • Resource requirements
  • Resource calendars
  • Project team assignments
  • Risk register
  • Lessons learned register

It provides information to:

  • Duration estimates

Duration estimates are an output from process 6.4 Estimate Activity Duration in the PMBOK® Guide – Sixth Edition. They are developed throughout the project as schedule and activity details are refined.

Document element Description
ID Unique identifier
Parametric estimates
Effort hours

Enter amount of labor it will take to accomplish the work. Usually shown in hours, but may also be shown in days.

Example: 150 hours

Resource quantity

Document the number of resources available.

Example: 2 people

Percent available

Enter amount of time the resources are available. Usually shown as the percent of time available per day or per week.

Example: 75 percent of the time

Performance factor  

Estimate a performance factor if appropriate. Generally effort hours are estimated based on the amount of effort it would take the average resource to complete the work.
This can be modified if you have a highly skilled resource or someone who has very little experience. The more skilled the resource, the lower the performance factor.
For example, an average resource would have a 1.0 performance factor.
A highly skilled resource could get the work done faster, so you multiply the effort hours times a performance factor of 0.8
A less skilled resource will take longer to get the work done, so you would multiply the effort hours times 1.2.

Example: A skilled worker with a performance factor of 0.8

Duration estimate  

Divide the effort hours by the resource quantity times the percent available times the performance factor to determine the length of time it will take to accomplish the work. The equation is:

Effort/ (quantity * percent available * performance factor) = duration Example: 150/ (2 * 0.75 * 0.8) = 125 hours

Analogous estimates
Previous activity

Enter a description of the previous activity.

Example: Build a 160 square foot deck.

Previous duration

Document the duration of the previous activity.

Example: 10 days

Current activity

Describe how the current activity is different.

Example: Build a 200 square foot deck.

Multiplier

Divide the current activity by the previous activity to get a multiplier.

Example: 200/160 = 1.25

Duration estimate

Multiply the duration for the previous activity by the multiplier to calculate the duration estimate for the current activity.

Example: 10 days * 1.25 = 12.5 days

Three-point estimate (beta distribution)
Optimistic duration

Determine an optimistic duration estimate. Optimistic estimates assume everything will go well and there won’t be any delays in material and that all resources are avail- able and will perform as expected.

Example: 20 days

Most likely duration

Determine a most likely duration estimate. Most likely estimates assume that there will be some delays but nothing out of the ordinary.

Example: 25 days

Pessimistic duration

Determine a pessimistic duration estimate. Pessimistic estimates assume there are significant risks that will materialize and cause delays.

Example: 36 days

Weighting equation

Weight the three estimates and divide. The most common method of weighting is the beta distribution:

tE = (tO + 4tM + tP)/6

Example: (20 + 4(25) + 36)/6

Expected duration

Enter the expected duration based on the beta distribution calculation.

Example: 26 days