What is the interquartile range of the data set

There are many methods of doing this. This one finds the median; however, instead of halving, you can use quarters instead.

First order the data, ascendingly:

10, 17, 18, 18, 18, 20, 20, 25, 25, 30, 30

Find the median:

For an even number of values: The number of values / 2

For an odd number of values: The number of values + 1 / 2

11 is odd.

11 + 1 / 2

12 / 2 = 6

The median is at the 6th position.

10, 17, 18, 18, 18, 20, 20, 25, 25, 30, 30

The median is 20.

Other method:

You don't need this step.

Using the values lower than the median, exclusive, find the median of the lower values;

For an odd number of values: The number of values + 1 / 2

10, 17, 18, 18, 18

5 + 1 / 2 = 6 / 2 = 3

The median of the lower values is at position 3.

This is the lower quartile.

10, 17, 18, 18, 18

Other method:

For an odd number of values: The number of values + 1 / 4

11 + 1 / 4 = 12 / 4 = 3

10, 17, 18, 18, 18, 20, 20, 25, 25, 30, 30

Using the values greater than the median, exclusive, find the median:

For an odd number of values: The number of values + 1 / 2

20, 25, 25, 30, 30

5 + 1 / 2 = 6 / 2 = 3

The median of the upper values is at position 3.

20, 25, 25, 30, 30

This is 25.

This is the upper quartile.

Other method:

For an odd number of values: (The number of values + 1 / 4) * 3

(11 + 1 / 4) * 3 = (12 / 4)  * 3 = 3 * 3 = 9

10, 17, 18, 18, 18, 20, 20, 25, 25, 30, 30

Your interquartile range is 3 - 25

25 - 3 = 21

1/800 is the right answer

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