Start project estimates with reference class forecasting…

When This Fires

• When beginning the estimation process for any multi-day or multi-week project
• When inside-view estimates feel precise but ungrounded in experience
• When projects in your area consistently overrun despite detailed bottom-up planning
• Complements Apply your personal estimation ratio as a multiplier… (personal estimation multiplier) with the project-level outside-view anchor

Common Failure Mode

The "this time is different" objection: "Past projects took 6 months, but this one has a simpler scope / better team / clearer requirements, so it will only take 3." Every project feels different from the inside. The reference class exists precisely because inside-view reasoning always finds reasons for optimism. Start with the base rate; then adjust for genuinely unique factors — but the adjustment should be modest, not halving the estimate.

The Protocol

(1) Before estimating the new project, identify 3-5 comparable past projects. Comparable means: similar scope, similar complexity, similar team size, similar domain. (2) Record their actual outcomes: how long did they take? What was estimated vs. actual? (3) Use the average actual duration as your anchor estimate — the starting point before any project-specific adjustments. (4) Now consider project-specific factors that genuinely differ from the reference class: simpler scope, more experienced team, tighter constraints. Adjust the anchor modestly (±20% maximum for each factor). (5) The final estimate should be between the reference-class average and your inside-view estimate, weighted toward the reference class. If the two diverge dramatically, trust the reference class — your inside view is probably doing exactly what the planning fallacy predicts.