When you studied cumulative frequency diagrams, you will have come across finding the interquartile range.
The reason we find the interquartile range is to give a measure of consistency between two sets of data.
If we knock out the top 25 % and the bottom 25% we are only looking at how spread out the central 50% of the data is. The lower this value, the more consistent the data is.
How to find the Quartiles.
Quartile just means quarter. If we are doing this in a small data set, we need to find the numbers that are in the position 1/4 and 3/4 of the way along this.
We find the position of the Lower and upper Quartiles by using the formula
LQ  = 

UQ  = 

Where n is the number of pieces of data in the list.
Example: If I had a list of 11 pieces of data.
The LQ would be the (11 + 1 / 4) 3rd number
The UQ would be the 3(11 + 1) / 4) 9th number.
Exampl2 1: Finding the IQR
What is the interquartile range of the data set
21, 23, 20, 27, 25, 24,, 30, 21,22, 28,30
Step 1: Order the data
20, 21, 21, 22, 23, 24, 25, 27, 28, 30, 30
Step 2: Find the position of the quartiles.
I have 11 pieces of data, this means the LQ is the 3rd number and the UQ is the 9th
Step 3: Find the value of the LQ and UQ
LQ = 3rd number = 21
UQ = 9th number = 28
Step 4: Find the IQR
IQR = UQ  LQ
= 28  21 = 7
Example 2: Finding the IQR from a frequency table
This frequency table shows the results of a survey into the number of people in a car that passes a point.
People in a car  1  2  3  4  5  6 
Frequency  45  198  121  76  52  13 
Find the IQR for this data.
Step 1: Find out the total frequency is for this problem,
We just add up all the frequencies for this. This gives us 505 cars
Step 2: Find the position of the quartiles.
Using the formula above, the LQ is the 126.5th number and the UQ is the 379.5th number
Step 3: Finding the values of the quartiles.
This is a bit complicated.
If I wrote the first column out as a list, I would write 45 numbers out. (have I gone past 126.5th yet?  No)
If I then write out the second column as a list I would have 45 + 198 = 243 numbers (Have I gone past 126.5th yet? Yes!)
My LQ must be in the second column so it must be 2
If you do the same for the UQ you find that UQ = 4
Step 4: Find the IQR
IQR = UQ  LQ = 4  2 = 2