Reference Class Forecasting

Using base rates and outside-view anchors to forecast more honestly

How do you make more accurate forecasts by using base rates instead of inside-view thinking?

Reference class forecasting, developed by Daniel Kahneman and Amos Tversky and formalized by Bent Flyvbjerg, improves forecast accuracy by anchoring on the statistical distribution of outcomes for similar past projects rather than on the details of the current one. The method reliably corrects the optimism bias that inflates cost, time, and benefit estimates in planning — the evidence base here is real and specific.

When people plan projects, they build a detailed narrative about their specific situation and forecast from it — the "inside view." Kahneman recognized that this process is reliably optimistic, because the narrative focuses on what could go right and ignores the distribution of outcomes for comparable projects. Reference class forecasting forces an anchor on that distribution before the inside-view details are added back. The practices below operationalize this outside-view discipline and are useful far beyond project management.