In Primavera P6, the “Duration Type” setting determines how the software calculates the duration and units of work for an activity. This setting is crucial for predicting the behavior of activities in terms of duration and resource units. Primavera P6 uses the following formulas to calculate the units of work:
Resource Units = Resource Units per Unit of Time * Duration
Remaining Resource Units = Resource Units per Unit of Time * Remaining Duration
Depending on the selected duration type, you can fix one or two elements of these equations, and Primavera P6 will calculate the remaining elements. The choice of which elements to fix depends on the nature of the work, the available information, the way it is progressed, and the schedule’s objectives. There are four types of activity durations in Primavera P6.
1. Fixed Duration and Units
This setting is used when both the duration and the number of resources are known and should remain fixed. With this configuration:
- Duration does not change when resources are added or removed or when Units/Time are changed.
- If the duration is changed, Units/Time will adjust, but the total Units remain unchanged.
2. Duration and Units/Time
This setting is used when both the duration and the throughput (Units/Time) of resources are known and must remain fixed. With this configuration:
- Duration does not change when resources are added or removed or when Units/Time change.
- If the duration is changed, the total Units will adjust, but the Units/Time remains unchanged.
- Primavera will maintain activity durations and increase work (Units), and thus the budget, based on additional amounts of work performed.
3. Fixed Units
This setting is used when the total amount of work required to complete an activity is fixed (e.g., excavating 50,000 m³ of earth). With this configuration:
- A change in Units/Time or Units will cause the activity duration to vary.
- If the duration is updated, the total Units remain unchanged, but the Units/Time will adjust.
- This type is recommended for activities where duration depends on the amount of resources.
4. Fixed Units/Time
This setting is used when an activity has a fixed productivity rate (Units/Time), regardless of the duration. With this configuration:
- The resource Units per Time remain unchanged while the activity duration or total Units change.
User Preferences
When assigning resources to an activity, user preferences in Primavera P6 affect how the duration, units, and units/time are calculated as additional resources are added. These preferences can be adjusted under Edit > User Preferences.
Preserve the Units, Duration, and Units/Time for existing assignments: With this option enabled, the duration remains constant when assigning resources, and units are calculated using the formula:
Units = Duration * Units/Time
Recalculate the Units, Duration, and Units/Time for existing assignments based on the activity Duration Type: With this option enabled, Primavera P6 will leave one or more parameters fixed and recalculate the rest based on the assigned duration type whenever a new resource is assigned. For example:
- An activity with fixed units will modify the duration when more resources are assigned.
- An activity with fixed duration and units will modify the units/time ratio of each resource and distribute the budgeted units among the resources.
By understanding and correctly setting the Duration Type, users can better control and predict the scheduling and resource allocation of activities in Primavera P6.