Percentile (and quartiles)
A percentile is the value a given share of the data falls at or below: 90% falls at or below the 90th. Quartiles are the 25th, 50th and 75th.
Select any words here to ask your Maestro about them.
On average, orders arrive in 32 minutes. The app promises 90% arrive in under 45. Does it keep that promise?
What it's for: saying where a value falls against all the others, without the extremes pulling it around. The 50th percentile is the median; the 25th and 75th are the first and third quartiles, the edges of the box in a box plot. The 90th, 95th and 99th percentiles are how you make promises about time: delivery, response, waiting.
How to read it: if your baby is in the 75th percentile for weight, they weigh as much as or more than 75 of every 100 babies their age. A percentile isn't a percentage of your data: the value at it is in the data's own units, pounds or minutes.
When your data has a tail, the mean and the median pull apart. A histogram shows you whether there's a tail.
Practice it in How do I read describe() for a column: mean versus median?.
How it's used
How to get it in Excel
The 200 times in B2:B201
- marked: =QUARTILE.INC(B2:B201,1)
- 21.65First quartile (25th percentile): a quarter of orders take less.
- =PERCENTILE.INC(B2:B201,0.5)
- 29.0550th percentile: the median. Same as =MEDIAN.
- =QUARTILE.INC(B2:B201,3)
- 39.05Third quartile (75th percentile): half the orders take between this and the first.
When to look closer
Percent correct isn't a percentile
A 120-question standardized test
- marked: Right answers: 74 of 120, or 62%
You got 62% of the test right. Did you do badly?
Ask your Maestro about Percentile
It answers from the free course page closest to this word, the way it answers while you study.
You: What does "Percentile" mean, in plain words?
Your Maestro: I'll explain it with a case of yours: it sticks better than a definition. Which number at work would read better as a percentile than as an average?
Asking about: Answers from the page "How do I count the missing values in each column with pandas?"
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Related terms
- Median
The median is the middle value when you sort the data from lowest to highest: half sit below it and half above. In Excel you get it with =MEDIAN(B2:B11).
- Box plot
A box plot sums up a variable in five numbers: the minimum, the first quartile, the median, the third quartile and the maximum. The box runs from the first to the third quartile and holds the middle half of the data.
- Histogram
A histogram shows how the values in a column of numbers are spread out: it cuts them into ranges of equal width and draws one bar per range, as tall as the number of values that fall in it.
- Mean (average)
To find the mean (the average), add up all the values and divide by how many there are: 15, 7, 8, 13 and 7 add up to 50, and 50 ÷ 5 = 10.