Originally published on Grammar Monkeys on Aug. 13, 2010.
Sometimes in journalism we have to — gasp! — do math, because it’s part of the news. We have to get the numbers right, just as we have to get the facts right and the language right. To those who think a journalism or communications degree means you can forget about math, think again: Math is a key part of many news stories, corporate memos, nonprofit reports, etc. And it needs to be done correctly.
So, in the spirit of the upcoming election season and its steady stream of polls, a note on the difference between “percent” and “percentage point.” This is also relevant when talking about tax rates, test scores, and so on.
Percent is a fraction of something. Percentage points are how percents are measured.
They are not the same thing, so if you are comparing two percents or rates, be careful how they are expressed. Here are a few examples:
- Roscoe P. Coltrane and Daisy Duke are running for mayor of Hazzard County. The latest poll shows 46 percent of likely voters choosing Roscoe and 41 percent for Daisy. It looks like Roscoe’s ahead by 5 percentage points (not 5 percent). But the margin of error of the poll is 4 percentage points, which means that either figure could be off by that much in either direction. So Roscoe may be further ahead than the poll indicates, as much as 50 to 37, or Daisy may actually be the one in the lead, with 45 percent to Roscoe’s 42. So in truth, we can’t say either one is ahead when it’s this close. Sometimes you hear the phrase “statistical dead heat” — this is what it means.
- 20 percent of the workers at the candy factory like the green gummy bears best, and 30 percent like the red ones best. The percents are a fraction of the total number of people asked. But when you want to know how many more people like red gummy bears than green ones, it gets tricky because you’re comparing rates, not raw numbers. So it’s not 10 percent more people that like red gummy bears, it’s 50 percent (30 minus 20 is 10, 10 divided by 20 is 0.5, or 50 percent), or you can say the red preference runs 10 percentage points higher than the green.
- In August 2007, unemployment was 4.1 percent. In August 2008, unemployment was 5.1 percent. How much did it go up? Not 1 percent, but 1 percentage point. The percent increase is actually 24.4 percent (5.1 minus 4.1 is 1, 1 divided by 4.1 is 0.244, or 24.4 percent).
- Sweet Valley High’s dropout rate is 6 percent; Ridgemont High’s is 8 percent. How much higher is Ridgemont’s rate? Not 2 percent, but 2 percentage points — or 33 percent — higher (8 minus 6 equals 2, 2 divided by 6 is 0.33, or 33 percent).