We all know (or should know) that when a project is running late, the predicted completion date calculated by the ‘critical path method’ (CPM) at an update tends to be optimistic, and this bias remains true for predictions based on simple time analysis as well as schedule calculations made using resource leveling.
There are two primary reasons for this:
- The assumption in CPM is that all future work will occur exactly as planned regardless of performance to date. The planned durations of future activities do not change.
- The burning of float has no effect of the calculated completion date until after the float is 100% consumed and the activity become critical.
For more on this issue see Why Critical Path Scheduling is Wildly Optimistic! [1]
Having an optimistic schedule for the motivation of resources to perform in not all bad – the updated CPM schedule shows the minimum level of performance needed to stop the situation deteriorating. The problem is more senior managers also need a reliable prediction of when the project can realistically be expected to finish and CPM cannot provide this. A more realistic / pessimistic view is obtained by apply the principles of Work Performance Management (WPM) to a CPM schedule, using ‘activity days’ taken from the CPM schedule as the metric.
Our latest article, WPM Solves CPM Optimism [2], uses a simple CPM schedule to demonstrate the differences in the calculated project completion dates between CPM and WPM. The value of WPM is stripping away the optimism bias inherent in CPM scheduling (particularly early in the project), thereby providing management with a clear indication of where the project is likely to finish if work continues at the current levels of productivity. These predictions are not a statement of fact, change the productivity and you change the outcome! A similar approach can be used to assess projected completion dates based on a simple manual bar chart.
To download the article, and see more on augmenting CPM with WPM to enhance controls information: https://mosaicprojects.com.au/PMKI-SCH-041.php#WPM-CPM [3]