Mental Model Fundamentals: Fermi Problem (Fermization)

Estimate approximately with little or no actual data before calculating precisely.

Fermi problems involve identifying key drivers of an estimate and making justified guesses about their relative quantities, including the most likely value, lower bound, and upper bound.

Related Examples:

• Fermi’s Piano Tuner - “Estimating the number of piano tuners in Chicago given only the population of the city.”
• Volume of Garbage - “What is the weight of garbage thrown away by each family every year?”
• Planes & Ping Pong Balls - Approximately how many ping pong balls would it take to fill a Boeing 747 airplane?

Related Quotes:

• “What Fermi understood is that by breaking down the question, we can better separate the knowable and the unknowable. So guessing—pulling a number out of the black box—isn’t eliminated. But we have brought our guessing process out into the light of day where we can inspect it. And the net result tends to be a more accurate estimate than whatever number happened to pop out of the black box when we first read the question.“ ~ Philip Tetlock

Related Concepts:

• Order of Magnitude - Estimating to the nearest power of ten can help make approximate comparisons.
• Confidence Interval - Using a sample, we can estimate the probability that the truth falls within a specific range.
• Occam’s Razor - Simpler explanations with fewer assumptions are more likely to be correct.
• Irreducibility - There is a lowest level of explanation and complexity beneath which a complete description is not possible.
• Proxy (Variable) - Something easily observable or measurable is often used in place of what is actually desired, despite not being directly relevant, as accurate, or as meaningful.
• Back-of-the-Envelope Calculation - “A rough calculation, typically jotted down on any available scrap of paper such as an envelope. It is more than a guess but less than an accurate calculation or mathematical proof. The defining characteristic of back-of-the-envelope calculations is the use of simplified assumptions.”
• Divide-and-Conquer Algorithm - “Recursively [break] down a problem into two or more sub-problems of the same or related type, until these become simple enough to be solved directly.”
• Chunking - “In cognitive psychology, chunking is a process by which individual pieces of an information set are broken down and then grouped together.”
• Recursion - “A method of solving a problem where the solution depends on solutions to smaller instances of the same problem.”
• Ansatz - “An educated guess or an additional assumption made to help solve a problem, and which is later verified to be part of the solution by its results.”

Related Resources:

• The Fermi Rule: Better be Approximately Right than Precisely Wrong
• Superforecasting: The Art and Science of Prediction

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