Just a few moments ago I read an other-wise helpful piece about recording formats, extolling the “exponential improvement” of CDs over cassette tapes as part of the background to the main story of HD-DVD vs BlueRay. It was the third time this weekend I’d come across similar usage, and the royal tummy churned each time. Am I the only one left that knows what the word means? Exponential changes are those that recur over time, with the same proportionate change in every time period. Compound interest at the bank is one, Moore’s Law (transistor count doubles every 18 months) another, and so is radioactive decay (half life).

Exponential is not a synonym for really, really, big. No one-time change can possibly be exponential, no matter how dramatic. If the proportion changes from one period to another (as in your assets being eroded by inflation) it isn’t exponential. That passbook savings rates are at absurdly low levels doesn’t change the fact that the pathetic balance is the result of exponential growth. If I Were King, educators who fostered the error would be picking lettuce and students would learn to use the word correctly, incidents like the ones I encountered this weekend would decline exponentially. Slowly, to be sure, but exponentially.