Estimating activity durations provides additional information, or confirmation, for cost estimates that may have been completed already. Understanding and agreeing upon likely activity durations contributes to developing your project schedule.
By the end of this topic, you will understand:
- The three types of activity duration
- Commonly used tools and techniques for estimating activity duration
- Activity duration estimates
- Program evaluation and review technique charts
- The relationship between numbers of resources and work efficiency.
The PMBOK Guide defines activity duration as the time between the start and end of a scheduled activity. Depending on the size of the project, the duration can be measured in days, weeks or months.
The following table details the three types of activity duration:
| Duration Type | Description |
|---|---|
| Actual duration |
|
| Original duration (OD) |
|
| Remaining duration |
|
Resource
Visit the following links to learn about activity durations and how to estimate them:
There are several tools or techniques used for estimating activity duration, including:
| Tool/Technique | Description |
|---|---|
| Expert judgement |
|
| Analogous estimation |
|
| Parametric estimating |
|
| Alternative analysis |
|
| Published estimating data |
|
| Vendor bid analysis |
|
| PERT Method |
|
As with other activity estimations discussed in this Learner Guide, you should, as project manager, seek input from people with experience in the work to be done as well as relevant experts.
Tips for estimating activity durations:
- Decide how accurate your estimate should be. More time will be spent on estimating if the project has to be estimated in detail. A less accurate estimate produced more quickly may be all that is necessary to be effective.
- Include contingency hours to reflect risk or uncertainty associated with any estimates.
Resource
Read about activity duration estimates at the following link:
‘Understanding Estimate Activity Durations in Project Management’ from Invensis
You can record duration estimates for use as a reference by using a template for activity duration estimates to create a table. This will provide input for developing the key deliverable from this phase: a PERT chart.
Resource
The following link includes for your use template for documenting activity duration estimates:
The key deliverable from PTM Phase 4 is a program evaluation and review technique (PERT) chart. This is similar to a network diagram discussed in section 3.4.
PERT charts are used to plan tasks within a project — making it easier to schedule and coordinate team members. …
PERT is similar to critical path in that they are both used to visualize the timeline and the work that must be done for a project. However, with PERT, you create three different time estimates for the project:
WrikeThe shortest possible amount of time each [activity or] task will take
The most probable amount of time
The longest time tasks might take if things don't go as planned.
This is a three-point estimate, similar to the method for estimating resources discussed in section 4.3. PERT is calculated backwards from a fixed end date for projects where contractor deadlines cannot be moved.
Developing a PERT Chart
A PERT chart plots the numbered tasks or events needed to complete a project on a timeline. The tasks are placed as nodes, which are linked by arrows. Task information in a PERT chart includes duration times and the critical path for the work.
Durations are an important input for developing the project schedule and can also be included in the project’s milestones. As project manager, you are responsible for these estimates, with input from key project team members or relevant experts.
Although there is a direct link between the amount of work needed and the time it may take, there is no direct relationship between the amount or number of resources assigned to an activity and its duration. However, adding more resources can be effective in shortening duration. This is called ‘crashing the activity’. This is effective only up to a certain point, called the ‘crash point’, where adding more resources increases the duration by decreasing efficiency and risks inconsistent work quality.
As project manager, you may need to consider the optimum loading, or most efficient number, of resources on a task.
Activity 5A: Estimating activity durations for FF
Case Study
Families First Relocation Project – Part 5
The Families First (FF) director has approved your linear responsibility chart and project budget. You explain that next, you will estimate durations for the relocation activities and confirm key dates for the project.
Read the case study ‘Families First Relocation Project – Part 5’. Then, prepare a PERT chart in a separate document or on a piece of paper for review and approval by the FF director.
Activity 5B: Estimating activity durations for your project
In a separate document or on a piece of paper, prepare a PERT chart and confirm the critical path activities for your project from previous Learning Activities.
