Concept: The P-Factor is like the wheel alignment on a car. For the car to run straight, the wheels must be properly aligned. Wheels are either in alignment or they are not. Although they can be wildly out of line, they cannot be better-than-aligned. The same limits apply to the P-Factor.
Figure 1
Practice: Recall that the P-Factor is a ratio: the total value actually earned in alignment with the schedule over the total value that should have been earned as scheduled. In both cases, the value is as of the ES Time. [1]
The ratio can be expressed succinctly as a formula:
EV denotes the value earned, PV denotes the value planned, and, most important, the subscripts “i” and “j” denote the target tasks. The target tasks for the PV are the set of tasks scheduled (S) for delivery (in full or in part) as of the ES Time. The target tasks for EV are a subset (“i”) of the “j” tasks, specifically, just the “j” tasks that have delivered value aligned (A) with the PVj as of the ES Time.
Now, consider what happens when there is Earned Value and all of it is aligned with the plan, i.e., j = i. Given that the two sets are identical and that the delivered value is the same as the planned value, the sum of EVj equals the sum of PVj, and the P-Factor equals 1.
Figure 2 illustrates such a case.
Figure 2
Unlike other ES metrics, the P-Factor can only go as high as 1. As suggested by the wheel analogy, you can’t top flawless alignment. Although this always happens at the end of a project [2], in practice, it rarely happens during the project.
At the other end of the spectrum, consider what happens when the value of all tasks is out of line with the plan. This is not unheard-of, especially in early stages of a project. This can occur because the value earned is 0 as of the ES Time. Less often, it’s because there is Earned Value, but it is not within the ES Time—it is early.
Figure 3 illustrates such a case.
Figure 3
Whether the EV is 0 or early, none of the EVj aligns with the schedule. So, the sum of EVj is 0. Assuming work is planned for the ES Time, the set of planned values is not empty. Given that PVj is non-zero, the P-Factor is 0.
Unlike other ES metrics [3], the P-Factor cannot go below 0. Returning to the wheel analogy, there can only be no alignment, i.e., P-Factor = 0. It cannot get worse than that.
By far the most common values of P-Factor are those that fall between the two limits. But, P-Factor’s maximum must be 1 and its minimum must be 0.
Notes:
[1] The ES Time is the time at which the value currently earned should have been earned.
[2] The sense in which all tasks are aligned at the end of a project will be discussed in a later post.
[3] The Schedule Variance for time (SVt) equals ES – AT and can, therefore, go below 0.
Glossary:
Actual Time = current period
Earned Value (EV) = aka, Budgeted Cost of Work Performed (BCWP)
ES = Earned Schedule--the time at which the value currently earned should have been earned
ES Time = Project Start Date + amount of Earned Schedule
P-Factor = a measure of schedule adherence
Planned Value (PV) = aka, Budgeted Cost of Work Scheduled (BCWS)
References:
Lipke, W. (2013). Schedule Adherence …a useful measure for project management. PM World Journal, Vol II, Issue VI.
Lipke, W. (2012). Schedule Adherence and Rework. CrossTalk, November-December.
Lipke, W. (2011b) Schedule Adherence and Rework. PM World Today, July.
Lipke, W. (2011a) Schedule Adherence and Rework. The Measurable News, Issue 1 (corrected version).
Lipke, W. (2009b). Earned Schedule. Lulu
Lipke, W. (2009a). Schedule Adherence …a useful measure for project management. The Measurable News, Issue 3.
Lipke, W. (2008). Schedule Adherence: AUseful Measure for Project Management. CrossTalk, April. |