Quartiles

Quartiles are the values that divide a list of numbers into quarters.

  • First put the list of numbers in order
  • Then cut the list into four equal parts
  • The Quartiles are at the "cuts"

Like this:

Example: 5, 8, 4, 4, 6, 3, 8

Put them in order: 3, 4, 4, 5, 6, 8, 8

Cut the list into quarters:

Quartiles

And the result is:

  • Quartile 1 (Q1) = 4
  • Quartile 2 (Q2), which is also the Median, = 5
  • Quartile 3 (Q3) = 8

Sometimes a "cut" is between two numbers ... the Quartile is the average of the two numbers.

Example: 1, 3, 3, 4, 5, 6, 6, 7, 8, 8

The numbers are already in order

Cut the list into quarters:

Quartiles

In this case Quartile 2 is half way between 5 and 6:

Q2 = (5+6)/2 = 5.5

And the result is:

  • Quartile 1 (Q1) = 3
  • Quartile 2 (Q2) = 5.5
  • Quartile 3 (Q3) = 7

Interquartile Range

The "Interquartile Range" is from Q1 to Q3:

Interquartile Range

To calculate it just subtract Quartile 1 from Quartile 3, like this:

Example:

Quartiles

The Interquartile Range is:

Q3 - Q1 = 8 - 4 = 4

Box and Whisker Plot

We can show all the important values in a "Box and Whisker Plot", like this:

Box and Whisker Plot

A final example covering everything:

Example: Box and Whisker Plot and Interquartile Range for

4, 17, 7, 14, 18, 12, 3, 16, 10, 4, 4, 11

Put them in order:

3, 4, 4, 4, 7, 10, 11, 12, 14, 16, 17, 18

Cut it into quarters:

3, 4, 4 | 4, 7, 10 | 11, 12, 14 | 16, 17, 18

In this case all the quartiles are between numbers:

  • Quartile 1 (Q1) = (4+4)/2 = 4
  • Quartile 2 (Q2) = (10+11)/2 = 10.5
  • Quartile 3 (Q3) = (14+16)/2 = 15

Also:

  • The Lowest Value is 3,
  • The Highest Value is 18

So now we have enough data for the Box and Whisker Plot:

And the Interquartile Range is:

Q3 - Q1 = 15 - 4 = 11