Percentages In Decisions

Sales tax, tips, discounts, interest rates, battery life, test scores — once you notice, percentages are running almost every number you meet in a day.

The useful habit is to read each one as a fraction of something. "15% APR" is a fraction of what you owe. "40% off" is a fraction of the sticker price. "Battery at 20%" is a fraction of a full charge. The percent is never the whole story until you know what it is a percent of.

Points Are Not Percent

Here is a trap that fools even careful readers. If approval rises from 40% to 50%, that is a 10 percentage-point gain — but it is a 25% increase, because 10 is a quarter of 40.

News headlines blur the two on purpose. "Support jumped 10%" and "support jumped 10 points" describe completely different changes. Always ask which one you are looking at: a gap measured in points, or a change measured in percent.