Concept: As described in the preceding post, there are Pro’s and Con’s to identifying problem tasks with Schedule Adherence (SA). A closer look at the problem tasks themselves suggests another set of Pro’s and Con’s.
As Walt Lipke pointed out, ES is the time at which the value currently earned should have been earned (Lipke, 2008). That makes the ES time a tipping point: the spot in the timeline where total Planned Value and total Earned Value are equal. By that time, each deliverable should have earned all and only the value planned for it. Any value earned outside that constraint does not adhere to the schedule.
There are only two possible ways for tasks to earn value outside the ES time. In common parlance, tasks are either early or late. While informative, the common terms are ambiguous. For instance, say a task earns value before it was scheduled and later requires rework. The same value is earned again. In this case, is value earned early or late?
As ES is a formal theory, it carefully circumscribes its terms to remove ambiguity. If a task fails to earn its initial planned value by the ES time, it is late. If a task earns value by the ES time and that is before it was planned to be earned, it is early. In ES, the former tasks are characterized as Impeded or Constrained (IC-tasks), and the latter tasks are characterized as premature and likely to incur Rework (R-tasks).
Practice: The distinction between R-tasks and IC-tasks spawns Pro’s and Con’s.
Pro: As described in last month’s post, SA identifies tasks that fail to adhere to the schedule. Those tasks are further characterized as R-tasks and IC-tasks.
The two sets of tasks are mutually exclusive and collectively exhaustive. How so? Tasks are either early, on schedule, or late. Formally, the categories are identified for each task (i) respectively, as follows: EVi@AT > PVi@ES, EVi@AT = PVi@ES, or EVi@AT < PVi@ES. [1] Numerically, the difference between the two terms (i.e., EVi@AT - PVi@ES) is either positive, zero, or negative. Each and every task in the schedule fits into one and only one of these spots. QED. (For a related formal proof, click here.)
With a difference of zero, on-schedule tasks do not adversely affect measures of schedule adherence. Thus, we can focus on the tasks that do: the R-tasks and IC-tasks. We often observe that the root causes of tasks in each group share commonalities. (For an example, click here .) That enables us to assess the impact of rework separately from that of impedance or constraint.
At the same time, we can weigh their combined effect on schedule performance. Again, we can explore the root causes for commonalities--this time, across categories.
The two different perspectives help us assess each category's separate contribution as well as their combined impact when deciding how to improve schedule performance.
Con: The downside of the R-task/IC-task distinction is in trade-offs. With two distinct and separate sets, it is tempting to trade off one against the other. For instance, one might want to trade-off working ahead (and facing rework) against not starting work (and facing impediments / constraints).
ES theory, however, demands a necessary balance between the value of R-tasks and IC-tasks. (For details, click here.) The necessary balance eliminates any real trade-off between the two sets: if the balance must always be maintained, you cannot sacrifice one without sacrificing the other.
That limits the distinction’s usefulness in practice. Worse, it conflicts with intuition. Instinctively, it seems that tasks done prematurely and ones impeded or constrained are independent of one another. So, a tight mathematical connection between them might appear to be without merit.
There are good reasons to challenge such a conclusion (for the counter-argument, click here ), but the intuitive conflict lingers.
Notes:
[1] EV = Earned Value, AT = Actual Time, PV = Planned Value (Lipke, 2011a).
References:
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). |
