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Inside Collection (Textbook):

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

# Practice 2: Spread of the Data

Summary: Practice exercise for Descriptive Statistics

## Student Learning Outcomes

• The student will calculate measures of the center of the data.
• The student will calculate the spread of the data.

## Given

The population parameters below describe the full-time equivalent number of students (FTES) each year at Lake Tahoe Community College from 1976-77 through 2004-2005. (Source: Graphically Speaking by Bill King, LTCC Institutional Research, December 2005).

Use these values to answer the following questions:

• μμ = 1000 FTES
• Median = 1014 FTES
• σσ = 474 FTES
• First quartile = 528.5 FTES
• Third quartile = 1447.5 FTES
• nn = 29 years

## Calculate the Values

### Exercise 1

A sample of 11 years is taken. About how many are expected to have a FTES of 1014 or above? Explain how you determined your answer.

6

### Exercise 2

75% of all years have a FTES:

• a. At or below:
• b. At or above:

• a. 1447.5
• b. 528.5

### Exercise 3

The population standard deviation =

474 FTES

### Exercise 4

What percent of the FTES were from 528.5 to 1447.5? How do you know?

50%

### Exercise 5

What is the IQRIQR? What does the IQRIQR represent?

919

### Exercise 6

How many standard deviations away from the mean is the median?

#### Solution

0.03

Additional Information: The population FTES for 2005-2006 through 2010-2011 was given in an updated report. (Source: http://www.ltcc.edu/data/ResourcePDF/LTCC_FactBook_2010-11.pdf). The data are reported here.

 Year 2005-06 2006-07 2007-08 2008-09 2009-10 2010-11 Total FTES 1585 1690 1735 1935 2021 1890

### Exercise 7

Calculate the mean, median, standard deviation, first quartile, the third quartile and the IQR. Round to one decimal place.

#### Solution

mean = 1809.3
median = 1812.5
standard deviation = 151.2
First quartile = 1690
Third quartile = 1935
IQR = 245

### Exercise 8

Construct a boxplot for the FTES for 2005-2006 through 2010-2011 and a boxplot for the FTES for 1976-1977 through 2004-2005.

### Exercise 9

Compare the IQR for the FTES for 1976-77 through 2004-2005 with the IQR for the FTES for 2005-2006 through 2010-2011. Why do you suppose the IQRs are so different?

#### Solution

Hint: Think about the number of years covered by each time period and what happened to higher education during those periods.

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