Fermi Estimation

The method is named after physicist Enrico Fermi, who liked asking students how many piano tuners work in Chicago. Nobody knows that number. But you can guess the city's population, the share of households with a piano, how often a piano is tuned and how many tunings one tuner manages a year. Multiply them and you land surprisingly close. It works for two reasons. First, splitting a big unknown into small ones turns a vague hunch into assumptions you can see and challenge. Second, independent guesses err in different directions, so their mistakes partly cancel. Philip Tetlock found that superforecasters do this constantly. Where possible, begin the chain with a base rate, meaning how often similar things usually happen. Only then adjust for your case's specifics. The goal is the right order of magnitude, not a precise figure.