If you take the lowest half of the data (from the lower extreme to the median) and find the median of these numbers, that is called the lower quartile or quartile 1.
If you take the upper half of the data (from the upper extreme to the median) and find the median of these numbers, that is called the upper quartile or quartile 3.
The interquartile range is the difference between quartile 1 and quartile 3.
4, 5, 6, 8, 9, 10, 11, 13
Median=8.5
Q1=6.5
Q3=10.5
10.5-6.5=4
I also added my PowerPoint if you didn't get what I was saying.
