 Motivation Problem

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Review Median

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First and Third Quartile

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Five-Number Summary

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Box-and-Whisker Plot

# First Quartile and Third Quartile

Definitions:

• The lower half of a data set is the set of all values that are to the left of the median value when the data has been put into increasing order.
• The upper half of a data set is the set of all values that are to the right of the median value when the data has been put into increasing order.
• The first quartile, denoted by Q1 , is the median of the lower half of the data set. This means that about 25% of the numbers in the data set lie below Q1 and about 75% lie above Q1 .
• The third quartile, denoted by Q3 , is the median of the upper half of the data set. This means that about 75% of the numbers in the data set lie below Q3 and about 25% lie above Q3 .

Example 1: Find the first and third quartiles of the data set {3, 7, 8, 5, 12, 14, 21, 13, 18}.

First, we write data in increasing order: 3, 5, 7, 8, 12, 13, 14, 18, 21. As on the previous page, the median is 12.

Therefore, the lower half of the data is: {3, 5, 7, 8}.

The first quartile, Q1, is the median of {3, 5, 7, 8}.

Since there is an even number of values, we need the mean of the middle two values to find the first quartile:  .

Similarly, the upper half of the data is: {13, 14, 18, 21}, so  .

Example 2: Find the first and third quartiles of the set {3, 7, 8, 5, 12, 14, 21, 15, 18, 14}.

Note that here we consider the two 14's to be distinct elements and not representing the same item; consider this like you obtained a score of 14 on two different quizzes.

First, we write the data in increasing order: 3, 5, 7, 8, 12, 14, 14, 15, 18, 21.

As before, the median is 13 (it is the mean of 12 and 14 — the pair of middle entries).

Therefore, the lower half of the data is: {3, 5, 7, 8, 12}.

Notice that 12 is included in the lower half since it is below the median value.

Then Q1 = 7 (there are five values in the lower half, so the middle value is the median). Similarly, the upper half of the data is: {14, 14, 15, 18, 21}, so Q3 = 15.

## Self Check Problem

The following dollar amounts were the hourly collections from a Salvation Army kettle at a local store one day in December: \$19, \$26, \$25, \$37, \$32, \$28, \$22, \$23, \$29, \$34, \$39, and \$31. Determine the first quartile and third quartile for the amount collected.

Solution 