Boxplots

Now, we will have a look at another specific type of chart called a boxplot. This kind of graph is used to display the distribution of a variable based on its quartiles. Quartiles are the values that split a dataset into quarters. Each quarter contains exactly 25% of the observations. For example, in the following sample data, the quartiles will be as follows:

Figure 10.36: Example of quartiles for the given data

So, the first quartile (usually referred to as Q1) is 4; the second one (Q2), which is also the median, is 5; and the third quartile (Q3) is 8.

A boxplot will show these quartiles but also additional information, such as the following:

• The interquartile range (or IQR), which corresponds to Q3 - Q1
• The lowest value, which corresponds to Q1 - (1.5 * IQR)
• The highest value, which corresponds to Q3 + (1.5 * IQR)
• Outliers, that is, any point outside of the lowest and highest points:

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