relative reduction

A relative reduction expresses a fall as a share of where it started. If a rate drops from 10 percent to 7 percent, it has fallen by 3 percentage points, but by 30 percent in relative terms, because 3 is 30 percent of 10.

The difference matters because the same absolute change means more when the starting level is low. Global health targets are often written this way. The World Health Organization's goal for tobacco called for a 30 percent relative reduction in the share of adults using tobacco between 2010 and 2025. From a 2010 level of 26.2 percent, that meant reaching 18.3 percent. WHO projected 19.2 percent for 2025, a gap of 0.9 percentage points and a relative reduction of about 27 percent.

When a story reports a percentage fall in a rate, it is worth checking which of the two is meant.