Use reference class forecasting for project estimates

Estimate how long similar projects have taken historically before estimating your specific project.

Why it works

Planning fallacy — the tendency to underestimate time and cost — is a direct application of base-rate neglect: people focus on the inside view (the specific plan) and ignore the outside view (how similar projects actually performed). Reference class forecasting corrects this by requiring the outside view first and treating the specific plan as an adjustment to the base rate, not a replacement for it.

How to do it

1. Identify the reference class: "What is the best comparison group for this project?"
2. Look up (or estimate) how long and at what cost the typical project in that class was delivered.
3. Use the historical distribution as your starting forecast; adjust only for specific, known differences.

Evidence

Reference class forecasting, formalized by Kahneman and further developed by Bent Flyvbjerg, has been validated on large infrastructure projects and is now a recommended forecasting method in several national planning standards. (observational)

Sources

• Flyvbjerg (2008), "Curbing Optimism Bias and Strategic Misrepresentation," European Planning Studies
• Kahneman & Lovallo (1993), "Timid Choices and Bold Forecasts: A Cognitive Perspective on Risk-Taking," Management Science

Common mistake

Choosing a flattering reference class ("similar projects by top teams") rather than the accurate one ("projects with similar scope and constraints, regardless of team").