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Descriptive Statistics: Measuring the Center of the Data

Module by: Susan Dean, Barbara Illowsky, Ph.D.. E-mail the authors

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Summary: This chapter discusses measuring descriptive statistical information using the center of the data

Note: Your browser may not currently support MathML. See our browser support page for additional details. You can always view the correct math in the PDF version.

The "center" of a data set is also a way of describing location. The two most widely used measures of the "center" of the data are the mean (average) and the median. To calculate the mean weight of 50 people, add the 50 weights together and divide by 50. To find the median weight of the 50 people, order the data and find the number that splits the data into two equal parts (previously discussed under box plots in this chapter). The median is generally a better measure of the center when there are extreme values or outliers because it is not affected by the precise numerical values of the outliers. The mean is the most common measure of the center.

The mean can also be calculated by multiplying each distinct value by its frequency and then dividing the sum by the total number of data values. The letter used to represent the sample mean is an xx with a bar over it (pronounced "xx bar"): x¯ x .

The Greek letter μμ (pronounced "mew") represents the population mean. If you take a truly random sample, the sample mean is a good estimate of the population mean.

To see that both ways of calculating the mean are the same, consider the sample:

1; 1; 1; 2; 2; 3; 4; 4; 4; 4; 4

x ¯ = 1 + 1 + 1 + 2 + 2 + 3 + 4 + 4 + 4 + 4 + 4 11 = 2.7 x ¯ = 1 + 1 + 1 + 2 + 2 + 3 + 4 + 4 + 4 + 4 + 4 11 =2.7(1)
x ¯ = 3 × 1 + 2 × 2 + 1 × 3 + 5 × 4 11 = 2.7 x ¯ = 3 × 1 + 2 × 2 + 1 × 3 + 5 × 4 11 =2.7(2)

In the second example, the frequencies are 3, 2, 1, and 5.

You can quickly find the location of the median by using the expression n + 1 2 n + 1 2 .

The letter nn is the total number of data values in the sample. If nn is an odd number, the median is the middle value of the ordered data (ordered smallest to largest). If nn is an even number, the median is equal to the two middle values added together and divided by 2 after the data has been ordered. For example, if the total number of data values is 97, then n + 1 2 n + 1 2 = 97 + 1 2 97 + 1 2 = 4949. The median is the 49th value in the ordered data. If the total number of data values is 100, then n + 1 2 n + 1 2 = 100 + 1 2 100 + 1 2 = 50.550.5. The median occurs midway between the 50th and 51st values. The location of the median and the median itself are not the same. The upper case letter MM is often used to represent the median. The next example illustrates the location of the median and the median itself.

Example 1

Problem 1

AIDS data indicating the number of months an AIDS patient lives after taking a new antibody drug are as follows (smallest to largest):

3; 4; 8; 8; 10; 11; 12; 13; 14; 15; 15; 16; 16; 17; 17; 18; 21; 22; 22; 24; 24; 25; 26; 26; 27; 27; 29; 29; 31; 32; 33; 33; 34; 34; 35; 37; 40; 44; 44; 47

Calculate the mean and the median.

Solution

The calculation for the mean is:

x ¯ = [ 3 + 4 + ( 8 ) ( 2 ) + 10 + 11 + 12 + 13 + 14 + ( 15 ) ( 2 ) + ( 16 ) ( 2 ) + ... + 35 + 37 + 40 + ( 44 ) ( 2 ) + 47 ] 40 = 23.6 x ¯ = [ 3 + 4 + ( 8 ) ( 2 ) + 10 + 11 + 12 + 13 + 14 + ( 15 ) ( 2 ) + ( 16 ) ( 2 ) + ... + 35 + 37 + 40 + ( 44 ) ( 2 ) + 47 ] 40 =23.6

To find the median, M, first use the formula for the location. The location is:

n + 1 2 = 40 + 1 2 = 20.5 n + 1 2 = 40 + 1 2 =20.5

Starting at the smallest value, the median is located between the 20th and 21st values (the two 24s):

3; 4; 8; 8; 10; 11; 12; 13; 14; 15; 15; 16; 16; 17; 17; 18; 21; 22; 22; 2424; 2424; 25; 26; 26; 27; 27; 29; 29; 31; 32; 33; 33; 34; 34; 35; 37; 40; 44; 44; 47

M = 24 + 24 2 = 24 M= 24 + 24 2 =24

The median is 24.

Example 2

Problem 1

Suppose that, in a small town of 50 people, one person earns $5,000,000 per year and the other 49 each earn $30,000. Which is the better measure of the "center," the mean or the median?

Solution

x ¯ = 5000000 + 49 × 30000 50 = 129400 x ¯ = 5000000 + 49 × 30000 50 =129400

M = 30000 M=30000

(There are 49 people who earn $30,000 and one person who earns $5,000,000.)

The median is a better measure of the "center" than the mean because 49 of the values are 30,000 and one is 5,000,000. The 5,000,000 is an outlier. The 30,000 gives us a better sense of the middle of the data.

Another measure of the center is the mode. The mode is the most frequent value. If a data set has two values that occur the same number of times, then the set is bimodal.

Example 3: Statistics exam scores for 20 students are as follows

Statistics exam scores for 20 students are as follows:

50 ; 53 ; 59 ; 59 ; 63 ; 63 ; 72 ; 72 ; 72 ; 72 ; 72 ; 76 ; 78 ; 81 ; 83 ; 84 ; 84 ; 84 ; 90 ; 93

Problem 1

Find the mode.

Solution

The most frequent score is 72, which occurs five times. Mode = 72.

Example 4

Five real estate exam scores are 430, 430, 480, 480, 495. The data set is bimodal because the scores 430 and 480 each occur twice.

When is the mode the best measure of the "center"? Consider a weight loss program that advertises an average weight loss of six pounds the first week of the program. The mode might indicate that most people lose two pounds the first week, making the program less appealing.

Statistical software will easily calculate the mean, the median, and the mode. Some graphing calculators can also make these calculations. In the real world, people make these calculations using software.

The Law of Large Numbers and the Mean

The Law of Large Numbers says that if you take samples of larger and larger size from any population, then the mean x ¯ x ¯ of the sample gets closer and closer to µµ. This is discussed in more detail in the section The Central Limit Theorem of this course.

Note:

The formula for the mean is located in the Summary of Formulas section course.

Glossary

Mean:
A number that measures the central tendency. A common name for mean is 'average.' The term 'mean' is a shortened form of 'arithmetic mean.' By definition, the mean for a sample (denoted by x¯ x ) is x¯ = Sum of all values in the sampleNumber of values in the sample x = Sum of all values in the sampleNumber of values in the sample size 12{ { bar {X}}= { {"Sum of all values in the sample"} over {"Number of values in the sample"} } } {}, and the mean for a population (denoted by μμ size 12{m} {}) is μ=Sum of all values in the populationNumber of values in the populationμ=Sum of all values in the populationNumber of values in the population size 12{m= { {"Sum of all values in the population"} over {"Number of values in the population"} } } {}.
Median:
A number that separates ordered data into halves. Half the values are the same number or smaller than the median and half the values are the same number or larger than the median. The median may or may not be part of the data.
Mode:
The value that appears most frequently in a set of data.

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