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. (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

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.