Take the outside view (base rates)

Estimate using the track record of similar cases, not the vivid details of your own.

Why it works

System 1 builds a confident story from the specific case in front of you ("the inside view") and ignores how often such cases actually succeed. Anchoring on base rates — how comparable projects or decisions turned out — corrects the planning fallacy and the systematic optimism that the inside view produces.

How to do it

1. Define a reference class: "projects like this one."
2. Find how that class actually performed — typical timelines, costs, success rates.
3. Start your estimate from that base rate, then adjust only modestly for specifics.

Evidence

The planning fallacy and the benefit of reference-class forecasting are well documented; base-rate neglect is one of the most replicated findings in judgment research. Buehler, Griffin, and Ross (1994) show experimentally that people underestimate their own completion times even while accurately judging others’, and Flyvbjerg (2006) demonstrates the correction at scale — reference-class forecasting measurably tightens the wildly optimistic cost and schedule estimates of large real-world projects. (observational)

A good reference class can be hard to define, and a wrong one imports the wrong base rate. Garbage class in, garbage forecast out.

Sources

• Kahneman & Tversky (1979), inside vs outside view and the planning fallacy
• Flyvbjerg, reference-class forecasting in large projects
• Buehler, R., Griffin, D., & Ross, M. (1994). Exploring the "planning fallacy": Why people underestimate their task completion times. Journal of Personality and Social Psychology, 67(3), 366–381.
• Flyvbjerg, B. (2006). From Nobel Prize to project management: Getting risks right. Project Management Journal, 37(3), 5–15.

Common mistake

Insisting "this time is different" because your case has special details. Almost everyone’s case has special details; the base rate already accounts for that.