Step by step
The question
Question: find the first quartile, Q1, of 12, 5, 8, 20, 15, 9, 11, 7 and 14.
Q1 is the 25th percentile.
About 25% of the numbers are below it.
1 Put them in order
In order, smallest to biggest.
Small green numbers count the positions: 1, 2, 3, and so on.
2 Find the position
The position i: 25/100, times n plus 1.
n is how many numbers: 9.
25/100 is 0.25.
9 plus 1 is 10.
0.25 times 10 is 2.5.
Careful!
The rule uses n plus 1, not n.
That keeps the middle in the middle.
3 Count to the position
2.5 is not whole.
It is between the 2nd and the 3rd numbers: 7 and 8.
So we take the number halfway between them: add, then divide by 2.
7 plus 8 is 15.
Divided by 2: 7.5.
4 Check
Let's check: 2 of the 9 numbers are below 7.5.
That is about 22%, close to 25%.
So it makes sense!
Let's look back.
First: put them in order.
Then: find the position.
Then: count to the position.
Last: check.