Example:

A box and whisker plot is used to display a set of data so that you can easily see where most of the numbers are.

For example, suppose you were to catch and measure the length of 13 fish in a lake:

A box and whisker plot is based on medians. The first step is to rewrite the data in order, from smallest length to largest:

Now find the median of all the numbers. Notice that since there are 13 numbers, the middle one will be the seventh number:

This must be the median (middle number) because there are six numbers on each side.

The next step is to find the lower quartile. This is the middle of the lower six numbers. The exact centre is half-way between 8 and 9 ... which would be 8.5

Now find the upper quartile. This is the middle of the upper six numbers. The exact centre is half-way between 14 and 14 ... which must be 14

Now you are ready to construct the actual box & whisker graph. First you will need to draw an ordinary number line that extends far enough in both directions to include all the numbers in your data:

First, locate the main median 12 using a vertical line just above your number line:

Now locate the lower quartile 8.5 and the upper quartile 14 with similar vertical lines:

Next, draw a box using the lower and upper median lines as endpoints:

Finally, the whiskers extend out to the data's smallest number 5 and largest number 20:

This is a box & whisker plot!

But what does it mean? What information about the data does this graph give you?

Well, it's obvious from the graph that the lengths of the fish were as small as 5 cm, and as long as 20 cm. This gives you the range of the data ... 15.
You also know the median, or middle value was 12 cm.
Since the medians (three of them) represent the middle points, they split the data into four equal parts. In other words:

The shading below, as an example, shows the quarter of the numbers that are between 12 and 14:

Here is a picture of the quarter of the data that is between 8.5 and 12. Notice that the data is more spread out here:

This picture is showing where half the data numbers are. Half of all the fish caught had a length between 8.5 and 14 centimetres:

Get the idea? See if you can answer the following question:

"Below what value is three quarters of the data?"

(Scroll down to see if you got it right!)
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The answer: "Three quarters of the data is below 14."