The Differences Between Microsoft Project and Primavera P6
In order for project managers to properly assign resources and develop workable schedules, they need to understand the difference between Duration, Effort, and Hours. Microsoft Project users often refer to these constraints as Duration, Units, and Work. Duration is the total amount of time that the task requires for successful completion, and is usually measured in days. Effort, Units, or possibly, staffing as it is sometimes referred to, is the amount of total available time a resource gives to a task, and is usually measured in hours per day or a percentage. Hours or Work is the amount of time required by the resource to actually do the task, and is measured in time units, such as hours.
Whether you say Duration, Effort, and Hours or Duration, Units, and Work it is important to understand the relationship between these elements. These elements balance out such that if one is changed, at least one of the others must be recalculated.
The formula that describes the relationship between Duration, Hours, and Effort (or Duration, Units, and Work) is below:
Duration x Effort = Hours
Duration x Units = Work
Another way of describing this equation combines the two:
Duration x Effort = Work
Below is a schematic of the combined equation.
The scheduling software expects the scheduler to provide two of the inputs and the software computes the third. Knowing which one of these elements is fixed is important, and it affects the duration type specified in Primavera P6 activities.
Duration (Duration = Work/Effort)
Task duration is a critical element of scheduling. Your project’s finish date is determined by the duration of the tasks making up the project. The duration of tasks is calculated from the estimated Work (Hours) divided by the assigned resource Units (Effort). As an example, if two people are assigned full time (on an eight hour daily schedule) to a four-day task (32-hours), the duration will be 32-hours Work divided by 16-hours/day Effort or 2-days. If the work is a two-day task (16-hours) performed by two labor resources (16-hours/day) then the corresponding duration would be one day.
If you decrease the duration of a task, then you must either reduce the total hours it was supposed to consume (Work), or increase the number of resources to complete the task in the time required (Effort). The more resources assigned to a task the shorter the duration. However, not all tasks can be reduced in duration by adding labor resources to the task. This is the reason it is important to correctly apply the appropriate Duration Type to the respective activity. It is beyond the scope of this article to discuss Duration Type in detail.
Work (Work = Duration x Effort)
Work is the product of duration and effort. Again, Work is the amount of time required by the resource to actually perform the task. As an example, two people are assigned full time (16-hours/day) to work on a task scheduled to be complete in six days. Work equals duration multiplied by effort, so 6-day x 16-hours/day equals 96-hours or 12-days of work, which will be completed by two resources. If the two labor resources are assigned to work half time on the task then 6-day x 8-hours/day equals 48-hours or 6-days of work.
Effort (Effort = Work/Duration)
In the last element case, completing six days of work in three days would require an effort by two resources working full time, i.e. 6-days or 48-hours divided by 16-hours/day is 3 days duration. As duration increases effort decreases. If the duration is decreased then the corresponding effort must increase.
Understanding the relationships between activity elements and the respective Work formula is important for accurately computing the Duration, Units, or Work (Duration, Effort, or Hours) on a project.
The constraints in Primavera P6 are termed slightly differently from Microsoft Project, but it still comes down to the three basic constraints of Duration, Effort, and Hours or Duration, Units, and Work. The definitions are essentially the same although termed slightly differently. Perhaps, the best terminology is the combination of the two: Duration, Effort, and Work. Knowing and specifying which of these three constraints are fixed is important for assigning resources and developing workable schedules.
Primavera P6 has four different duration types (Note that Microsoft Project only has three): Fixed Duration and Units, Fixed Duration and Units/Time, Fixed Units, and Fixed Units/Time. These settings are four different ways that Primavera P6 determines the relationship between the activity duration, work (Units), and the resources assigned or effort (Units/Time). Microsoft Project has three different duration types: Fixed Duration, Fixed Units, and Fixed Work.