The plan said six months but the project took ten months. Why didn’t the plan say ten months? That seems a reasonable question to ask of whoever crafted the plan.
Every week, somebody publishes a list of the things that make projects late and over budget. All of these lists agree on the significant issues.
These are not unknowns. They can be listed, their impacts quantified, probabilities attached, and baked into the plan.
I’ve never seen a plan that included a factor for scope creep, or the sum of the many hazards that could bring down the development server, or flu season, etc. Why not?
At the very least: “Even though our bottom-up CPM analysis says six months, all the similar projects we could find took between ten and twelve months, so we’re estimating eleven months.”
How can we make sure we give projects the time and resources they need to succeed if we propose hopelessly optimistic plans that ignore significant risk factors?