Schedule · tool 7 of 7

Reference Class Forecasting

Reference Class Forecasting: corrects optimism by using what happened on similar projects.

Best for: Estimating large or unfamiliar work · Time: 1–3 hours
Domain: Schedule · Focus area: Planning

How it works

People estimate from the inside, picturing how the work should go. This is the planning fallacy. Reference class forecasting takes the outside view: look at how comparable projects actually turned out, and start there.

Choose a class of truly similar projects.
Use their actual results, not their plans.
Adjust only for documented differences.

How to apply it

Find five to ten comparable projects.
Record planned versus actual time and cost.
Take the typical overrun.
Apply it to your estimate before adjusting.

In practice

Your last eight website launches overran by 15–40%. A 10-week plan is more honestly 12–14 weeks.

Pro tip

Keep a simple record of estimates against actuals. In a year it will beat anyone’s judgment.

Use it

Case study: Berlin Brandenburg Airport
Case study: Crossrail (Elizabeth line)

Quick check

What does the outside view rely on?

Sources

D. Lovallo and D. Kahneman, “Delusions of Success”, Harvard Business Review (2003)
B. Flyvbjerg and D. Gardner, How Big Things Get Done (2023)

Last reviewed 2026-11-02 against the PMBOK® Guide, 8th edition · Practitioner review pending · How this guide is made