Duration Type controls the way Primavera P6 manages the variables of activity duration, effort, and/or budget when adding multiple resources to an activity. It also affects these same variables when progressing the schedule with updates.
When a schedule is in the planning phase it is typically a good idea to set the duration type to fixed duration and units/time for most situation This is done so that Primavera P6 honors your activity duration estimates and increases the unknown work (units) and, therefore, the budget, based on additional required resource effort (units/time).
In other words choosing Fixed Duration and Units/Time locks the activity’s duration, and the default units/time (effort) values for each resource added, while allowing the overall unit cost to increase with each assignment. Once you have built out your resource loaded schedule it is quite legitimate to change the Duration Type setting for your activities to change this behavior once the project gets underway.
An example of when you might do this is when you are under some external direction in the way you build and maintain a project schedule. For example, when progressing a schedule the United States Army Corps of Engineer (USACE) requirements say to set the duration type to fixed duration and units.
Fixed units makes sense because once the budget is set it is very difficult to get additional cost increases approved. Fixed duration is good because schedules are typically time constrained and have been carefully developed to demonstrate how the work sequence will allow the achievement of all major contractual milestones in the schedule.
So for time and budget constrained projects set the duration type to fixed duration and units before making schedule updates. Setting the duration type improperly comes with undesired affects.
The remainder of this article demonstrates the effect of duration type on activity remaining duration, remaining units/time, and at completion units when progressing the schedule with updates. Note that this article only considers the physical percent complete type.
Our demonstration project is displayed in Figure 1.
This Primavera P6 schedule performs three activities in parallel. The percent complete type of all activities is physical. The duration type for tasks A, B, and C is fixed duration and units/time, fixed duration and units, and fixed units, respectively. The fixed units/time duration type is not addressed. Let’s consider each duration type’s impact on schedule updates.
Fixed Duration and Units/Time
The physical percent complete type requires more manual input than the duration percent complete type. It is also considered more accurate as it accounts for varying production rate due, possibly, e.g. to learning curves. It allows this by separating the percent complete value from the remaining duration value; so for example you can be the 50% mark in terms of elapsed time, but have only achieve 25% of the physical work required to complete the task – hence Physical Percent Compete.
In Figure 2, bottom details status tab, we specify in the physical % cell that the activity is 80% complete.
We then have to either specify an expected finish date or remaining duration. In our example we enter a remaining duration of 2-days.
Now in the bottom details resources tab, Figure 3, we find a budgeted units of 40-hours, and actual and remaining units of 24-hour and 16-hour respectively for each resource.
Apparently, P6 calculated a 3-days completed units because of the 2-days remaining specified on the activity. Well, this was not our intention when stating that the activity is 80% complete. We wanted P6 to say the actual units is 32-hours or 80% of the allotted time. So in Figure 4 we enter our desired actual units of 32-hours for each resource.
Well, the remaining units and remaining units/time remain constant, as per our fixed duration and units/time duration type. The only variable then is work and the at completion units increases to 48-hours, and the at completion cost also increases. Because units was not fixed our schedule does not honor the agreed upon budget, which is not good.
Fixed Duration and Units
We again enter the same progress update; the only difference now is the duration type is fixed duration and units. We set the physical % complete to 80% and the remaining duration, again, to 2-days, Figure 5.
P6 again computes from these values actual units of 24-hours, and remaining units of 16-hours, Figure 6.
We adjust the actual units of each resource to 32-hours. Now watch what happens, Figure 7.
The remaining units decreases to 8-hours and the remaining units/time to 4-hour/day. This is the desired effect. We have completed 80% of the work, so only 8-hours remains. But we have specified 2-days remaining duration on activity B. P6 lowers to production rate to spread the work over the specified 2-days and to maintain the at completion units of 40-hours. This is what we want!
Now with only units (work) fixed we enter the same activity progress, Figure 8.
This time P6 computes actual units of 0-hours, Figure 9, which is disconcerting.
We again enter the desired 32-hour actual units. When we do this unusual computations result, Figure 10.
The remaining units is 8-hours, good. But the remaining units/time increases to 20-hours/day, confusing. Further, when we display the bottom details status tab, Figure 11, we find that our activity duration compressed to 0.4-days, not good.
The units (work) remained fixed at 40-hours, but to achieve this work value P6 significantly increased the effort and significantly compressed to activity duration. This is all very confusing.
Figure 12 displays the final schedule after moving the data date forward one week and recalculating the schedule.
The remaining duration of activities A and B with fixed durations is 2-days. Activity C does not fix duration, and the resulting duration decreases to 0.4-days: not what we want. We definitely want to fix the duration. Fixing units (work) is also preferred because projects become cost constrained after budget approval.
As demonstrated, the Primavera P6 duration type affects not only assignment of multiple resources, but also schedule progress. Inmost cases therefore, it is recommended that you use fixed duration and units/time in the planning phase of your project.
Fixed duration and units is the preferred duration type for progressing the schedule; projects in the execution phase tend to be both time constrained and budget constrained. The variable is the daily effort, which adjusts, accordingly, to fill the allotted time and spend the agreed budget.