Step 5: Estimate Activity Durations
Activity durations should be realistic, achievable, and most likely — with no padding for margin or risk (risk and contingency are handled separately later in the planning process).
Duration Principles
- Activity durations should be as short as possible to assist with the measurement of accomplished work.
- Durations in the short and medium term generally should not exceed the program updating period (e.g. 4 weeks).
- Long-duration activities should be broken into shorter activities where practical. Exceptions include procurement of long lead-time items and equipment.
- Programs with a large ratio of very short-duration activities (1 day or less) may be too detailed and will make the program difficult to update and manage.
Methods for Estimating Activity Durations
First Principles
Measured quantities × reliable productivity rates. Recommended when sufficient design information is known.
Benchmarking & Historical Data
Data from similar project types. Useful for early estimates when design detail is limited.
Experience
Planner or team member knowledge from delivering similar work.
Suppliers' Quotes
Delivery lead times and installation commitments from suppliers and subcontractors.
Plug Estimates
Rough order-of-magnitude estimates used when limited information is available. Should be replaced as the design develops.
Recommended approach:
A first-principles approach using measured quantities, productivity rates, resource/plant availability, and knowledge of construction methodologies is recommended when sufficient design information is known.
Consult the Team
Project team experts and activity owners should be consulted when estimating durations — they often have the best knowledge of how long activities will take in practice.
- For example: the Design Manager will be able to give accurate estimates for how long each stage of design development and approvals are likely to take.
All assumptions — productivity rates, benchmark data, and expert advice — should be documented in the Basis of Schedule.
Resource Impacts on Duration
- The number of resources available (labour and non-labour) can affect estimates of work and activity durations.
- Where possible, significant resources should be assigned to the program to determine whether all required resources will be available when needed, identify peak resource demands, and ensure resources can be managed effectively.
- Any resources assigned to the program should be cross-checked against resources allocated in the project budget.