Incident: A stock index that chopped itself in half
6 min read·14 Oct 2026
Module 2 · Incident file · About 25 minutes
What happened
In 1982 the Vancouver Stock Exchange in Canada started a new share index: one number meant to show, at a glance, how the prices on the exchange were moving. A computer recalculated it thousands of times a day.
Each time, the new value was truncated to three decimal places. Truncating means cutting off the digits you do not keep, instead of rounding to the nearest value. Every cut was tiny, a fraction of a thousandth. By 1983 the index had drifted to about half the value it should have had. Then it was corrected, and the index went back up to its true level. The case was widely reported at the time, for example by The Wall Street Journal in November 1983.
What was lost was the one thing an index is for: a number people could trust.