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What is a Decile?

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What is a Decile?

Deciles are similar to quartiles. But while quartiles sort data into four quarters, deciles sort data into ten equal parts: The 10th, 20th, 30th, 40th, 50th, 60th, 70th, 80th, 90th and 100th percentiles.

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A decile rank assigns a number to a decile:

Decile Rank Percentile
1 10th
2 20th
3 30th
4 40th
5 50th
6 60th
7 70th
8 80th
9 90th

The higher your place in the decile rankings, the higher your overall ranking. For example, if you were in the 99th percentile for a particular test, that would put you in the decile ranking of 10. A person who scored very low (say, the 5th percentile) would find themselves in a decile rank of 1.

what is a decile

A chart showing decile rankings for discharged stroke patients. Image: SUNY Buffalo

Why are Decile ranks used instead of percentiles of quartiles?

Decile rankings are just another way to categorize data. Which system you use is usually a judgment call. For example, if you wanted to display class rankings on a pie chart, using deciles would make more sense that percentiles. That’s because a pie chart with 10-categories would be much easier to read than a pie chart with 99 categories.

What is a Decile used for in Real Life?

Deciles and decile ranks are used more often in real life than in the classroom. For example, Australia uses decile ranks to report drought data. Deciles 1-2 represent the lowest 20% (“much below normal”). That means droughts that are “much below normal” don’t occur more than 20% of the time.

Deciles are also commonly used for college admissions and high school rankings. For example, this chart from Roanoke College shows the high school decile rankings for the student body.

Next: Interdecile Range

Stephanie Glen. "What is a Decile?" From Elementary Statistics for the rest of us!

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