MEPX
Chapter 5 of 10All chapters
  1. 1Why estimate
  2. 2Anchors worth knowing
  3. 3Fermi estimates
  4. 4Rounding well
  5. 5Sanity checks
  6. 6Estimating percentages
  7. 7Estimating time
  8. 8Making big numbers real
  9. 9Where estimates go wrong
  10. 10Getting better

Chapter 5 of 10

Sanity checks

Testing an answer you already have.

Per unit

Divide the total by the number of people, days or items and ask whether that per-unit figure is believable. Almost every absurd statistic collapses under this one move.

  • Check units on both sides: they must match or something is wrong.
  • Compare against an anchor you trust rather than against your expectation.

Sense of scale

A million seconds is about eleven days; a billion is about 32 years. Holding that difference in mind makes large financial and population figures readable.