Box and Whisker Plots
A Box and Whisker plots shows the median, the quartiles, and the extremes
for a numerical set of data. The box portion contains about 50% of the data
values. The two whiskers each contain about 25% of the data values. It
shows how spread out the data values are. They are useful for comparing
sets of data.
How to make a box and whisker plot:
Example: Suppose you have the following prices of Microwave ovens (in dollars)
225 257 175 300 265 185 229 235 299
Step 1: Place the data in order from least to greatest:
175 185 225 229 235 257 265 299 300
Step 2: Find the lower extreme: 175
Find the upper extreme: 300
These are the end points of your whiskers
Step 3: Find the median of your data: 235
Step 4: Draw a line through your median. Then split the two halves in half again. If
there is an even number of data on each side you need to average the two middle numbers
to find the end of the lower quartile and the beginning of the upper quartile. See the
example below:
175 185
225 229 235 257 265
median
This is the halfway
point – Average these
two numbers to get the
end of the first quartile
185  225
 205
2
299 300
This is the halfway point –
Average these two numbers
to get the beginning of the
last quartile.
265  299
 282
2
Step 5: Graph the median, the extremes and the quartiles below a number line. Then
draw the box and whiskers.
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