1. Measures of Central Tendency
8.2
Compute the mean, median, and mode of distributions.
Find the five-number summary of a distribution.
Apply measures of central tendency to compare data.

2. The Mean and the Median
We use the Greek letter Σ (capital sigma) to indicate a sum. For example, we will write the sum of the data values 7, 2, 9, 4, and 10 by Σx =
We represent the mean of a sample of a population by x (read as “x bar”), and we will use the Greek letter μ (lowercase mu) to represent the mean of the whole population.
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Section 15.2, Slide 2

3. The Mean and the Median
Example: A car company has been studying its safety record at a factory and found that the number of accidents over the past 5 years was 25, 23, 27, 22, and 26. Find the mean annual number of accidents for this 5-year period.
Solution: We add the number of accidents and divide by 5.
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Section 15.2, Slide 3

4. The Mean and the Median
Example: The water temperature at a point downstream from a plant for the last 30 days is summarized in the table. What is the mean temperature for this distribution?
(continued on next slide)
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Section 15.2, Slide 4

5. The Mean and the Median
Solution: A third column is added to the table that contains the products of the raw scores and their frequencies.
The mean is
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Section 15.2, Slide 5

6. The Mean and the Median
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Section 15.2, Slide 6

7. The Mean and the Median
Example: Listed are the yearly earnings of some celebrities.
a) What is the mean of the earnings of the celebrities on this list?
b) Is this mean an accurate measure of the “average” earnings for these celebrities?
(continued on next slide)
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Section 15.2, Slide 7

8. The Mean and the Median
Solution (a): Summing the salaries and dividing by 10 gives us
Solution (b): Eight of the celebrities have earnings below the mean, whereas only two have earnings above the mean. The mean in this example does not give an accurate sense of what is “average” in this set of data because it was unduly influenced by higher earnings.
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Section 15.2, Slide 8

9. The Mean and the Median
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Section 15.2, Slide 9

10. The Mean and the Median
Example: The table lists the ages at inauguration of the presidents who assumed office between 1901 and Find the median age for this distribution.
Solution: We first arrange the ages in order to get
There are 17 ages. The middle age is the ninth, which is 55.
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Section 15.2, Slide 10

11. The Mean and the Median
Example: Fifty 32-ounce quarts of a particular brand of milk were purchased and the actual volume determined. The results of this survey are reported in the table. What is the median for this distribution?
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Section 15.2, Slide 11

12. The Mean and the Median
Solution: Because the 50 scores are in increasing order, the two middle scores are in positions 25 and 26. We see that 29 ounces is in position 25 and 30 ounces is in position 26. The median for this distribution is
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Section 15.2, Slide 12

13. The Five Number Summary
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Section 15.2, Slide 13

14. The Five Number Summary
Example: Consider the list of ages of the presidents from a previous example:
42, 43, 46, 51, 51, 51, 52, 54, 55,
55, 56, 56, 60, 61, 61, 64, 69.
Find the following for this data set:
a) the lower and upper halves
b) the first and third quartiles
c) the five-number summary
(continued on next slide)
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Section 15.2, Slide 14

15. The Five Number Summary
Solution:
(a): Finding the median, we can identify the lower and upper halves.
(b): The median of the lower half is
The median of the upper half is
(continued on next slide)
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Section 15.2, Slide 15

16. The Five Number Summary
(c): The five number summary is
We represent the five-number summary by a graph called a box-and-whisker plot.
(continued on next slide)
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Section 15.2, Slide 16

17. The Five Number Summary
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Section 15.2, Slide 17

18. The Five Number Summary
Example: Find the mode for each data set.
a) 5, 5, 68, 69, 70
b) 3, 3, 3, 2, 1, 4, 4, 9, 9, 9
c) 98, 99, 100, 101, 102
d) 2, 3, 4, 2, 3, 4, 5
Solution: a) The mode is 5.
b) There are two modes: 3 and 9.
In c) and d) there is no mode.
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Section 15.2, Slide 18

19. Comparing Measures of Central Tendency
Example: Assume that you are negotiating the contract for your union. You have gathered annual wage data and found that three workers earn $30,000, five workers earn $32,000, three workers earn $44,000, and one worker earns $50,000. In your negotiations, which measure of central tendency should you emphasize?
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Section 15.2, Slide 19

20. Comparing Measures of Central Tendency
Solution:
Mode: $32,000
Median: $32,000
Mean:
The mean is $36,000.
To make the salaries appear as low as possible, you would want to use the mode and median.
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Section 15.2, Slide 20