About/Fermi Estimation

Skill 04 / 06

Fermi Estimation.

Order-of-magnitude reasoning, structured estimation, sanity checks. Graded on the path you took to the number.

What gets tested.

Easy is the definition. Normal combines two. Hard hides which one applies until you've done the work.

The decomposition.

Breaking the problem into multiplicative components. The right factor tree gets you 80% of the way. The wrong one is the wrong answer.

Anchoring.

Using known reference values: city populations, hours/year, the constants you should know cold.

Sanity checks.

Comparing the estimate to bounds you know. If McDonald’s revenue is > world GDP, recheck.

Confidence intervals.

Reporting a range, not a point. Width × accuracy, both graded.

Cross-checks.

Two paths to the same estimate. Reconcile the gap.

Bad assumptions.

The one factor that swings the estimate by an order of magnitude.

“Right number, wrong path.
No offer.”

Take a drill.

Other skills

Five more to sharpen.

Train them in any order. The system targets your level in each.

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