The Wall Street Journal uses the word “percentile” incorrectly
Subscribe to Decision Science News by Email (one email per week, easy unsubscribe)
OOPS
So, we’re on the Twitter and we see this animated GIF from the Wall Street Journal. It says
- $53,647 is the 24th percentile of Americans in 2014
- $53,647 is the 15th percentile among blacks
- $53,647 is the 13th percentile among black women
- $53,647 is the 29th percentile among black women with college degrees
Wait. What?
It says $54k is the 24th percentile of Americans. That means 24% of Americans earn less than $54k. The rest (76%) earn more. Americans are wealthier than I thought.
Then says $54k is the 13th percentile among black women. That means 13% of black women earn less than $54k. The rest (87%) earn more. However, according to the graphic, only 71% of black women with college degrees make more than $54k. So the subset with college degrees earns less than those with or without college degrees?
None of this makes any sense, unless of course you are using the word “percentile” incorrectly. The Xth percentile of a distribution is a value such at X% of the population is at or below that value. If your test score is at the 99th percentile, you’re one percentage point from the top and 99 percentage points from the bottom. If you score is at the first percentile, you’re one percentage point from the bottom and 99 percentage points from the top. Here’s a refresher on percentiles.
The WSJ used the term percentile backwards (100 minus percentile instead of percentile) in their Twitter infographic and they made the same mistake on their website in their Real Time Economics piece What Percent Are You?
Have a look below. With the slider moved to $100k income, the graphic reads “8th percentile”. Nope. That’s the 92nd percentile. 8th percentile: relatively poor. 92nd percentile: relatively wealthy.
The trouble comes, we think from them wanting to say “1 percenter” for the richest one percent. But that doesn’t make it the 1st percentile, which is the lowest income hundredth of the population.
Maybe you don’t believe us because we’re just a crummy little blog and they’re the Wall Street Journal. So we’ll turn it over to the dictionaries:
a value on a scale of 100 that indicates the percent of a distribution that is equal to or below it
.
American Heritage Dictionary of Student Science:
The percentile of a given value is determined by the percentage of the values that are smaller than that value. For example, a test score that is higher than 95 percent of the other scores is in the 95th percentile.
.
Random House Kernerman Webster’s College Dictionary:
Ninety percent of the values lie at or below the ninetieth percentile, ten percent above it.
.
And just to make sure it’s not an Americanism, here are two English dictionaries from England where they invented English:
The 90th percentile is the value of a variable such that 90% of the relevant population is below that value.
.
Overall these students rank in the 21st percentile on the tests—that is, they did worse than 79 per cent of all children taking the test.
