The range tells you how spread out all the data values are. The interquartile range tells you the spread of the middle 50% of values.**Quartiles**

Quartiles divide the data into four equal groups.

If you have a data set with n values and put the data in ascending order then:

Q1 position (the lower quartile) = (n+1)/4

Q2 Position (the median) = 2(n+1)/4

Q3 Position (the upper quartile) = 3(n+1)/4**Example:** What is the upper quartile of the values 3,4,6,9,30,54,87

There are 7 values overall so the position of the upper quartile is 3(7+1)/4 = 6

So the upper quartile is the number in position 6.

i.e. the upper quartile is 54.**Interquartile Range**

This is just the difference between the upper and lower quartile. It contains the middle 50% of values.**Stem and Leaf Diagrams**

You might be asked to put values into a stem and leaf diagram before finding the the median etc. This is easy to do…just make something like the diagram below (just remember to include a key).**Box Plots**

These give you a summary of the spread of data and look like this:**Comparing Data**

If asked to compare two sets of data do the following:

- Look at the mean, media or mode – say which data set has the highest value and what this means in the context of the data (e.g. older people are generally heavier).
- Look at the spread – the smaller this is the smaller the variation in the data.