Use Accomplishment Value to Supplement Float Analysis
Accomplishment Value, or Earned Value (a.k.a. Budgeted Cost of Work Performed) pertains to the measurement of accomplishment against the plan, once the work is underway. Normally, when we have a critical path schedule, we rely primarily on total float to see how we're doing. If we maintain the desired float, we assume that the schedule progress is satisfactory. This can be a false security. If you read the manuals on the use of the critical path method, you probably were told that CPM analysis helps you to focus on the work that is most critical. The problem is that we can be watching the critical items while the rest of the project gets less emphasis on maintaining schedule.
Using CPM based models and relying primarily on total float (slack) to have us focus on the critical work can actually lead to schedule slippage rather than preventing it. This problem occurs when we concentrate so much on the activities that lie on the critical path that much of the other work does not get done when planned. Eventually, all of the work loses its float and there is no time margin left to deal with any problems that might crop up.
I monitor this by doing a periodic float analysis, to see if more and more tasks are joining the "low-float" group. The more tasks there are that have low float, the less room there is to manipulate the work to adjust for problems. This float analysis can be a very important early warning system.
But what if you do not have a critical path schedule, or do not feel comfortable with the float measurements? Are there other ways to measure progress and schedule risk? One alternative is to use Earned Value.
The EV method requires that we assign some kind of weight factor to each task. This can be resource hours, or budgeted cost. Or it can be an arbitrary estimate that allows us to balance the value of each task against the others.