The Customer Service Survey

Percentage Points

by Peter Leppik on Fri, 2011-12-02 15:20

Percentage PointsToday's XKCD comic captures one of my own pet peeves: sloppiness around percentages vs. percentage points.

A percentage change is a relative measurement, as compared to some other value. If Grayton started at 20% support in the polls, and his support drops by 19%, that means he's now at a little over 16% support. 19% of 20% is 3.8 percentage points, and 20% - 3.8% = 16.2%.

A percentage point change is an absolute difference in the value being measured. If Grayton's support drops by 19 percentage points, that means he went from 20% to 1%. That 19-point drop is a 95% drop in his support.

From the context of the news, we can surmise that the reporter probably meant that Grayton's support dropped by 19 percentage points. Unless, that is, a substantial number of voters are anti-Christmas and pro-drunk driving.

Confusingly, the media often uses the "percentage change" language to mean both a percentage change and a change of a certain number of percentage points. Reporters covering political polls will often say a candidate's support has change by so many percent when they mean percentage points. Medical stories often use the correct meaning, which also happens to sound more impressive: reducing heart attacks from 1% of patients to 0.5% of patients is a 50% drop in heart attacks.

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