RTConfidence, Inc.
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7. The Integrated Master Schedule (IMS) Critical Path May be Misleading.

Signing up to a schedule based on the project’s IMS critical path, without any buffer, is usually a losing proposition, especially if the project is complex and team performance is highly scrutinized.  This topic is treated in detail within my book, “Project Risk Management: A Practical Implementation Approach.”   Several factors tend to skew our schedules to the right.  The primary ones are: (1) when we look at “best case”, “nominal” and “worst case” task durations (where “nominal” is used to construct our IMS), the “worst case” is typically further from “nominal” than the “best case” is – probabilistically speaking, this gives the “nominal” duration less than 50% confidence, thus if most task durations follow this pattern, the percentage confidence for the overall project can be much lower than 50%; (2) iterations in activities such as design/build/test cycles typically have probabilities associated with subsequent “spins”, and our IMS does not use probabilities, thus we have to guess the number of spins and this is usually optimistic (especially if you are attempting to “do more with less”); and (3) there is a phenomena referred to as “merge bias” which is associated with the impact of multiple concurrent near-critical paths – which basically decreases the percentage confidence (sometimes significantly, especially on very complex projects) of meeting the IMS time-line.  The effects of these three factors alone can very likely put your schedule confidence into the single digits, and cannot be effectively detected and quantified in an IMS analysis unless you employ Modelling and Simulation techniques.