I am monitoring a schedule right now for a project that is about 57% complete (duration). Looking at it at two different perspectives--Earned Schedule and Critical Path--I am finding results that differ quite substantially. I know no predictive model is 100% valid and no two models will produce the exact same results but I was surprised by the degree of difference.
The schedule is a simple water fall, duration-based schedule that was constructed reasonably well, in that it is a constraint free schedule with no orphaned work packages and most of those work packages are decomposed where no duration is greater than two reporting periods. The way progress has developed, the remaining work packages are all on the critical path and the schedule is showing a five day unfavorable finish variance.
Using Earned Schedule, the project is showing over 10 business days late, close to 12, and using the estimate at completion formula the project is likely to be over a month late.
Work is probabilistic so I know that the five days late (CP) or the over a month late (ES) are two points within a range of likely results but I am surprised at the size of the variance between two predictions.
Has anyone seen this type of a discrepancy between two models before? Could this simply be due to the sensitivity of the models and, if so, which model do you believe is more valid / accurate?
Thanks for the help!