Module 11: The Chi Square Distribution
Section Exercises
Barbara Illowsky & OpenStax et al.
Facts About the Chi-Square Distribution
3. When does the chi-square curve approximate a normal distribution?
4. Where is μ located on a chi-square curve?
5. Is it more likely the df is 90, 20, or two in the graph?
Decide whether the following statements are true or false.
6. As the number of degrees of freedom increases, the graph of the chi-square distribution looks more and more symmetrical.
8. The mean and the median of the chi-square distribution are the same if df = 24.
Goodness-of-Fit Test
10. An economist is deriving a model to predict outcomes on the stock market. He creates a list of expected points on the stock market index for the next two weeks. At the close of each day’s trading, he records the actual points on the index. He wants to see how well his model matched what actually happened.
Use the following information to answer the next five exercises: A teacher predicts that the distribution of grades on the final exam will be and they are recorded in the table.
| Grade | Proportion |
|---|---|
| A | 0.25 |
| B | 0.30 |
| C | 0.35 |
| D | 0.10 |
The actual distribution for a class of 20 is in the table below.
| Grade | Frequency |
|---|---|
| A | 7 |
| B | 7 |
| C | 5 |
| D | 1 |
12. df= ______
13. State the null and alternative hypotheses.
Use the following information to answer the next nine exercises: The following data are real. The cumulative number of AIDS cases reported for Santa Clara County is broken down by ethnicity as in the table below.
| Ethnicity | Number of Cases |
|---|---|
| White | 2,229 |
| Hispanic | 1,157 |
| Black/African-American | 457 |
| Asian, Pacific Islander | 232 |
| Total = 4,075 |
The percentage of each ethnic group in Santa Clara County is as inthe table below.
| Ethnicity | Percentage of total county population | Number expected (round to two decimal places) |
|---|---|---|
| White | 42.9% | 1748.18 |
| Hispanic | 26.7% | |
| Black/African-American | 2.6% | |
| Asian, Pacific Islander | 27.8% | |
| Total = 100% |
20. Is this a right-tailed, left-tailed, or two-tailed test?
22. χ2 test statistic = _______
23. p-value = _______
24. Graph the situation. Label and scale the horizontal axis. Mark the mean and test statistic. Shade in the region corresponding to the p-value.
Let α = 0.05
- Decision: ________________
- Reason for the Decision: ________________
- Conclusion (write out in complete sentences): ________________
25. Does it appear that the pattern of AIDS cases in Santa Clara County corresponds to the distribution of ethnic groups in this county? Why or why not?
For each problem, use a solution sheet to solve the hypothesis test problem. Go to Appendix E for the chi-square solution sheet. Round expected frequency to two decimal places.
| Face Value | Frequency | Expected Frequency |
|---|---|---|
| 1 | 15 | |
| 2 | 29 | |
| 3 | 16 | |
| 4 | 15 | |
| 5 | 30 | |
| 6 | 15 |
27. The marital status distribution of the U.S. male population, ages 15 and older, is as shown in the table below.
| Marital Status | Percent | Expected Frequency |
|---|---|---|
| never married | 31.3 | |
| married | 56.1 | |
| widowed | 2.5 | |
| divorced/separated | 10.1 |
Suppose that a random sample of 400 U.S. young adult males, 18 to 24 years old, yielded the following frequency distribution. We are interested in whether this age group of males fits the distribution of the U.S. adult population. Calculate the frequency one would expect when surveying 400 people. Fill in the table, rounding to two decimal places.
| Marital Status | Frequency |
|---|---|
| never married | 140 |
| married | 238 |
| widowed | 2 |
| divorced/separated | 20 |
Use the following information to answer the next two exercises: The columns in the table below contain the Race/Ethnicity of U.S. Public Schools for a recent year, the percentages for the Advanced Placement Examinee Population for that class, and the Overall Student Population. Suppose the right column contains the result of a survey of 1,000 local students from that year who took an AP Exam.
| Race/Ethnicity | AP Examinee Population | Overall Student Population | Survey Frequency |
|---|---|---|---|
| Asian, Asian American, or Pacific Islander | 10.2% | 5.4% | 113 |
| Black or African-American | 8.2% | 14.5% | 94 |
| Hispanic or Latino | 15.5% | 15.9% | 136 |
| American Indian or Alaska Native | 0.6% | 1.2% | 10 |
| White | 59.4% | 61.6% | 604 |
| Not reported/other | 6.1% | 1.4% | 43 |
30. The City of South Lake Tahoe, CA, has an Asian population of 1,419 people, out of a total population of 23,609. Suppose that a survey of 1,419 self-reported Asians in the Manhattan, NY, area yielded the data in the table below. Conduct a goodness-of-fit test to determine if the self-reported sub-groups of Asians in the Manhattan area fit that of the Lake Tahoe area.
| Race | Lake Tahoe Frequency | Manhattan Frequency |
|---|---|---|
| Asian Indian | 131 | 174 |
| Chinese | 118 | 557 |
| Filipino | 1,045 | 518 |
| Japanese | 80 | 54 |
| Korean | 12 | 29 |
| Vietnamese | 9 | 21 |
| Other | 24 | 66 |
Use the following information to answer the next two exercises: UCLA conducted a survey of more than 263,000 college freshmen from 385 colleges in fall 2005. The results of students’ expected majors by gender were reported in The Chronicle of Higher Education (2/2/2006). Suppose a survey of 5,000 graduating females and 5,000 graduating males was done as a follow-up last year to determine what their actual majors were. The results are shown in the tables below. The second column in each table does not add to 100% because of rounding.
31. Conduct a goodness-of-fit test to determine if the actual college majors of graduating females fit the distribution of their expected majors.
| Major | Women – Expected Major | Women – Actual Major |
|---|---|---|
| Arts & Humanities | 14.0% | 670 |
| Biological Sciences | 8.4% | 410 |
| Business | 13.1% | 685 |
| Education | 13.0% | 650 |
| Engineering | 2.6% | 145 |
| Physical Sciences | 2.6% | 125 |
| Professional | 18.9% | 975 |
| Social Sciences | 13.0% | 605 |
| Technical | 0.4% | 15 |
| Other | 5.8% | 300 |
| Undecided | 8.0% | 420 |
32. Conduct a goodness-of-fit test to determine if the actual college majors of graduating males fit the distribution of their expected majors.
| Major | Men – Expected Major | Men – Actual Major |
|---|---|---|
| Arts & Humanities | 11.0% | 600 |
| Biological Sciences | 6.7% | 330 |
| Business | 22.7% | 1130 |
| Education | 5.8% | 305 |
| Engineering | 15.6% | 800 |
| Physical Sciences | 3.6% | 175 |
| Professional | 9.3% | 460 |
| Social Sciences | 7.6% | 370 |
| Technical | 1.8% | 90 |
| Other | 8.2% | 400 |
| Undecided | 6.6% | 340 |
Read the statement and decide whether it is true or false.
34. In general, if the observed values and expected values of a goodness-of-fit test are not close together, then the test statistic can get very large and on a graph will be way out in the right tail.
35. Use a goodness-of-fit test to determine if high school principals believe that students are absent equally during the week or not.
36. The test to use to determine if a six-sided die is fair is a goodness-of-fit test.
37. In a goodness-of fit test, if the p-value is 0.0113, in general, do not reject the null hypothesis.
| Business Type | Number in class | Observed Number that recycle one commodity | Expected number that recycle one commodity |
|---|---|---|---|
| Office | 35 | 19 | 17.5 |
| Retail/Wholesale | 48 | 27 | 24 |
| Food/Restaurants | 53 | 35 | 26.5 |
| Manufacturing/Medical | 52 | 21 | 26 |
| Hotel/Mixed | 24 | 9 | 12 |
| Age Class (Years) | Obese (Percentage) | Expected USA average (Percentage) |
|---|---|---|
| 20–30 | 75.0 | 32.6 |
| 31–40 | 26.5 | 32.6 |
| 41–50 | 13.6 | 36.6 |
| 51–60 | 21.9 | 36.6 |
| 61–70 | 21.0 | 39.7 |
Test of Independence
Determine the appropriate test to be used in the next three exercises.
Use the following information to answer the next seven exercises: Transit Railroads is interested in the relationship between travel distance and the ticket class purchased. A random sample of 200 passengers is taken. The table below shows the results. The railroad wants to know if a passenger’s choice in ticket class is independent of the distance they must travel.
| Traveling Distance | Third class | Second class | First class | Total |
|---|---|---|---|---|
| 1–100 miles | 21 | 14 | 6 | 41 |
| 101–200 miles | 18 | 16 | 8 | 42 |
| 201–300 miles | 16 | 17 | 15 | 48 |
| 301–400 miles | 12 | 14 | 21 | 47 |
| 401–500 miles | 6 | 6 | 10 | 22 |
| Total | 73 | 67 | 60 | 200 |
44. H0: _______
45. Ha: _______
47. How many passengers are expected to travel between 201 and 300 miles and purchase second-class tickets?
48. How many passengers are expected to travel between 401 and 500 miles and purchase first-class tickets?
50. What is the p-value?
Use the following information to answer the next eight exercises: An article in the New England Journal of Medicine, discussed a study on smokers in California and Hawaii. In one part of the report, the self-reported ethnicity and smoking levels per day were given. Of the people smoking at most ten cigarettes per day, there were 9,886 African Americans, 2,745 Native Hawaiians, 12,831 Latinos, 8,378 Japanese Americans and 7,650 whites. Of the people smoking 11 to 20 cigarettes per day, there were 6,514 African Americans, 3,062 Native Hawaiians, 4,932 Latinos, 10,680 Japanese Americans, and 9,877 whites. Of the people smoking 21 to 30 cigarettes per day, there were 1,671 African Americans, 1,419 Native Hawaiians, 1,406 Latinos, 4,715 Japanese Americans, and 6,062 whites. Of the people smoking at least 31 cigarettes per day, there were 759 African Americans, 788 Native Hawaiians, 800 Latinos, 2,305 Japanese Americans, and 3,970 whites.
52. Complete the table.
| Smoking Level Per Day | African American | Native Hawaiian | Latino | Japanese Americans | White | TOTALS |
|---|---|---|---|---|---|---|
| 1-10 | ||||||
| 11-20 | ||||||
| 21-30 | ||||||
| 31+ | ||||||
| TOTALS |
53. State the hypotheses.
54. H0: _______
55. Ha: _______
Calculate the following values:
59. p-value = ______
60. Is this a right-tailed, left-tailed, or two-tailed test? Explain why.
62. State the decision and conclusion (in a complete sentence) for the following preconceived levels of α.
α = 0.05
- Decision: ___________________
- Reason for the decision: ___________________
- Conclusion (write out in a complete sentence): ___________________
- Decision: ___________________
- Reason for the decision: ___________________
- Conclusion (write out in a complete sentence): ___________________
For each problem, use a solution sheet to solve the hypothesis test problem. Go to Appendix E for the chi-square solution sheet. Round expected frequency to two decimal places.
64. A recent debate about where in the United States skiers believe the skiing is best prompted the following survey. Test to see if the best ski area is independent of the level of the skier.
| U.S. Ski Area | Beginner | Intermediate | Advanced |
|---|---|---|---|
| Tahoe | 20 | 30 | 40 |
| Utah | 10 | 30 | 60 |
| Colorado | 10 | 40 | 50 |
65. Car manufacturers are interested in whether there is a relationship between the size of car an individual drives and the number of people in the driver’s family (that is, whether car size and family size are independent). To test this, suppose that 800 car owners were randomly surveyed with the results in the table. Conduct a test of independence.
| Family Size | Sub & Compact | Mid-size | Full-size | Van & Truck |
|---|---|---|---|---|
| 1 | 20 | 35 | 40 | 35 |
| 2 | 20 | 50 | 70 | 80 |
| 3–4 | 20 | 50 | 100 | 90 |
| 5+ | 20 | 30 | 70 | 70 |
| Major | < $50,000 | $50,000 – $68,999 | $69,000 + |
|---|---|---|---|
| English | 5 | 20 | 5 |
| Engineering | 10 | 30 | 60 |
| Nursing | 10 | 15 | 15 |
| Business | 10 | 20 | 30 |
| Psychology | 20 | 30 | 20 |
67. Some travel agents claim that honeymoon hot spots vary according to age of the bride. Suppose that 280 recent brides were interviewed as to where they spent their honeymoons. The information is given in Table. Conduct a test of independence.
| Location | 20–29 | 30–39 | 40–49 | 50 and over |
|---|---|---|---|---|
| Niagara Falls | 15 | 25 | 25 | 20 |
| Poconos | 15 | 25 | 25 | 10 |
| Europe | 10 | 25 | 15 | 5 |
| Virgin Islands | 20 | 25 | 15 | 5 |
68. A manager of a sports club keeps information concerning the main sport in which members participate and their ages. To test whether there is a relationship between the age of a member and his or her choice of sport, 643 members of the sports club are randomly selected. Conduct a test of independence.
| Sport | 18 – 25 | 26 – 30 | 31 – 40 | 41 and over |
|---|---|---|---|---|
| racquetball | 42 | 58 | 30 | 46 |
| tennis | 58 | 76 | 38 | 65 |
| swimming | 72 | 60 | 65 | 33 |
| Type of Fries | Northeast | South | Central | West |
|---|---|---|---|---|
| skinny fries | 70 | 50 | 20 | 25 |
| curly fries | 100 | 60 | 15 | 30 |
| steak fries | 20 | 40 | 10 | 10 |
70. According to Dan Lenard, an independent insurance agent in the Buffalo, N.Y. area, the following is a breakdown of the amount of life insurance purchased by males in the following age groups. He is interested in whether the age of the male and the amount of life insurance purchased are independent events. Conduct a test for independence.
| Age of Males | None | < $200,000 | $200,000–$400,000 | $401,001–$1,000,000 | $1,000,001+ |
|---|---|---|---|---|---|
| 20–29 | 40 | 15 | 40 | 0 | 5 |
| 30–39 | 35 | 5 | 20 | 20 | 10 |
| 40–49 | 20 | 0 | 30 | 0 | 30 |
| 50+ | 40 | 30 | 15 | 15 | 10 |
71. Suppose that 600 thirty-year-olds were surveyed to determine whether or not there is a relationship between the level of education an individual has and salary. Conduct a test of independence.
| Annual Salary | Not a high school graduate | High school graduate | College graduate | Masters or doctorate |
|---|---|---|---|---|
| < $30,000 | 15 | 25 | 10 | 5 |
| $30,000–$40,000 | 20 | 40 | 70 | 30 |
| $40,000–$50,000 | 10 | 20 | 40 | 55 |
| $50,000–$60,000 | 5 | 10 | 20 | 60 |
| $60,000+ | 0 | 5 | 10 | 150 |
Read the statement and decide whether it is true or false.
72. The number of degrees of freedom for a test of independence is equal to the sample size minus one.
| U.S. region/Flavor | Strawberry | Chocolate | Vanilla | Rocky Road | Mint Chocolate Chip | Pistachio | Row total |
|---|---|---|---|---|---|---|---|
| West | 12 | 21 | 22 | 19 | 15 | 8 | 97 |
| Midwest | 10 | 32 | 22 | 11 | 15 | 6 | 96 |
| East | 8 | 31 | 27 | 8 | 15 | 7 | 96 |
| South | 15 | 28 | 30 | 8 | 15 | 6 | 102 |
| Column Total | 45 | 112 | 101 | 46 | 60 | 27 | 391 |
77. The table provides a recent survey of the youngest online entrepreneurs whose net worth is estimated at one million dollars or more. Their ages range from 17 to 30. Each cell in the table illustrates the number of entrepreneurs who correspond to the specific age group and their net worth. Are the ages and net worth independent? Perform a test of independence at the 5% significance level.
| Age Group Net Worth Value (in millions of US dollars) | 1–5 | 6–24 | ≥25 | Row Total |
|---|---|---|---|---|
| 17–25 | 8 | 7 | 5 | 20 |
| 26–30 | 6 | 5 | 9 | 20 |
| Column Total | 14 | 12 | 14 | 40 |
| Opinion/Ethnicity | Asian-American | White/Non-Hispanic | African-American | Latino | Row Total |
|---|---|---|---|---|---|
| Against tax | 48 | 433 | 41 | 160 | 628 |
| In Favor of tax | 54 | 234 | 24 | 147 | 459 |
| No opinion | 16 | 43 | 16 | 19 | 84 |
| Column Total | 118 | 710 | 71 | 272 | 1171 |
Test for Homogeneity
79. A math teacher wants to see if two of her classes have the same distribution of test scores. What test should she use?
Use the following information to answer the next five exercises: Do private practice doctors and hospital doctors have the same distribution of working hours? Suppose that a sample of 100 private practice doctors and 150 hospital doctors are selected at random and asked about the number of hours a week they work. The results are shown in the table.
| 20–30 | 30–40 | 40–50 | 50–60 | |
|---|---|---|---|---|
| Private Practice | 16 | 40 | 38 | 6 |
| Hospital | 8 | 44 | 59 | 39 |
84. State the null and alternative hypotheses.
85. df = _______
88. What can you conclude at the 5% significance level?
For each word problem, use a solution sheet to solve the hypothesis test problem. Go to Appendix E for the chi-square solution sheet. Round expected frequency to two decimal places.
| Open | Conscientious | Extrovert | Agreeable | Neurotic | |
| Business | 41 | 52 | 46 | 61 | 58 |
| Social Science | 72 | 75 | 63 | 80 | 65 |
90. Do men and women select different breakfasts? The breakfasts ordered by randomly selected men and women at a popular breakfast place is shown in the table. Conduct a test for homogeneity at a 5% level of significance.
| French Toast | Pancakes | Waffles | Omelettes | |
| Men | 47 | 35 | 28 | 53 |
| Women | 65 | 59 | 55 | 60 |
91. A fisherman is interested in whether the distribution of fish caught in Green Valley Lake is the same as the distribution of fish caught in Echo Lake. Of the 191 randomly selected fish caught in Green Valley Lake, 105 were rainbow trout, 27 were other trout, 35 were bass, and 24 were catfish. Of the 293 randomly selected fish caught in Echo Lake, 115 were rainbow trout, 58 were other trout, 67 were bass, and 53 were catfish. Perform a test for homogeneity at a 5% level of significance.
92. In 2007, the United States had 1.5 million homeschooled students, according to the U.S. National Center for Education Statistics. In the table below you can see that parents decide to homeschool their children for different reasons, and some reasons are ranked by parents as more important than others. According to the survey results shown in the table, is the distribution of applicable reasons the same as the distribution of the most important reason? Provide your assessment at the 5% significance level. Did you expect the result you obtained?
| Reasons for Homeschooling | Applicable Reason (in thousands of respondents) | Most Important Reason (in thousands of respondents) | Row Total |
|---|---|---|---|
| Concern about the environment of other schools | 1,321 | 309 | 1,630 |
| Dissatisfaction with academic instruction at other schools | 1,096 | 258 | 1,354 |
| To provide religious or moral instruction | 1,257 | 540 | 1,797 |
| Child has special needs, other than physical or mental | 315 | 55 | 370 |
| Nontraditional approach to child’s education | 984 | 99 | 1,083 |
| Other reasons (e.g., finances, travel, family time, etc.) | 485 | 216 | 701 |
| Column Total | 5,458 | 1,477 | 6,935 |
| Year | European Union | United States | Row Total |
|---|---|---|---|
| 2010 | 3,413 | 7,164 | 10,557 |
| 2009 | 3,302 | 7,057 | 10,359 |
| 2008 | 3,505 | 7,488 | 10,993 |
| 2007 | 3,537 | 7,758 | 11,295 |
| 2006 | 3,595 | 7,697 | 11,292 |
| 2005 | 3,613 | 7,847 | 11,460 |
| Column Total | 45,011 | 20,965 | 65,976 |
| Year Car Type | Small | Mid-Size | Large | Small SUV | Mid-Size SUV | Large SUV | Row Total |
|---|---|---|---|---|---|---|---|
| 2009 | 12 | 22 | 10 | 10 | 27 | 6 | 87 |
| 2013 | 31 | 30 | 19 | 11 | 29 | 4 | 124 |
| Column Total | 43 | 52 | 29 | 21 | 56 | 10 | 211 |
Comparison of the Chi-Square Tests
99. How are tests of independence similar to tests for homogeneity?
For each word problem, use a solution sheet to solve the hypothesis test problem. Go to Appendix E for the chi-square solution sheet. Round expected frequency to two decimal places.
Read the statement and decide whether it is true or false.
103.
- Explain why a goodness-of-fit test and a test of independence are generally right-tailed tests.
- If you did a left-tailed test, what would you be testing?
Test of a Single Variance
104. What type of test should be used?
106. Is this a right-tailed, left-tailed, or two-tailed test?
Use the following information to answer the next three exercises: The standard deviation of heights for students in a school is 0.81. A random sample of 50 students is taken, and the standard deviation of heights of the sample is 0.96. A researcher in charge of the study believes the standard deviation of heights for the school is greater than 0.81.
Use the following information to answer the next four exercises: The average waiting time in a doctor’s office varies. The standard deviation of waiting times in a doctor’s office is 3.4 minutes. A random sample of 30 patients in the doctor’s office has a standard deviation of waiting times of 4.1 minutes. One doctor believes the variance of waiting times is greater than originally thought.
115. A sample standard deviation of 15 minutes is the same as a sample variance of __________ minutes.
117. H0: __________
118. df = ________
119. chi-square test statistic = ________
120. p-value = ________
121. Graph the situation. Label and scale the horizontal axis. Mark the mean and test statistic. Shade the p-value.
122. Let α = 0.05
- Decision: ________
- Conclusion (write out in a complete sentence.): ________
123. How did you know to test the variance instead of the mean?
124. If an additional test were done on the claim of the average delay, which distribution would you use?
125. If an additional test were done on the claim of the average delay, but 45 flights were surveyed, which distribution would you use?
For each word problem, use a solution sheet to solve the hypothesis test problem. Go to Appendix E for the chi-square solution sheet. Round expected frequency to two decimal places.
126. A plant manager is concerned her equipment may need recalibrating. It seems that the actual weight of the 15 oz. cereal boxes it fills has been fluctuating. The standard deviation should be at most 0.5 oz. In order to determine if the machine needs to be recalibrated, 84 randomly selected boxes of cereal from the next day’s production were weighed. The standard deviation of the 84 boxes was 0.54. Does the machine need to be recalibrated?
129. Airline companies are interested in the consistency of the number of babies on each flight, so that they have adequate safety equipment. They are also interested in the variation of the number of babies. Suppose that an airline executive believes the average number of babies on flights is six with a variance of nine at most. The airline conducts a survey. The results of the 18 flights surveyed give a sample average of 6.4 with a sample standard deviation of 3.9. Conduct a hypothesis test of the airline executive’s belief.
130. The number of births per woman in China is 1.6 down from 5.91 in 1966. This fertility rate has been attributed to the law passed in 1979 restricting births to one per woman. Suppose that a group of students studied whether or not the standard deviation of births per woman was greater than 0.75. They asked 50 women across China the number of births they had had. The results are shown in the table below. Does the students’ survey indicate that the standard deviation is greater than 0.75?
| # of births | Frequency |
|---|---|
| 0 | 5 |
| 1 | 30 |
| 2 | 10 |
| 3 | 5 |
131. According to an avid aquarist, the average number of fish in a 20-gallon tank is 10, with a standard deviation of two. His friend, also an aquarist, does not believe that the standard deviation is two. She counts the number of fish in 15 other 20-gallon tanks. Based on the results that follow, do you think that the standard deviation is different from two? Data: 11; 10; 9; 10; 10; 11; 11; 10; 12; 9; 7; 9; 11; 10; 11
132. The manager of “Frenchies” is concerned that patrons are not consistently receiving the same amount of French fries with each order. The chef claims that the standard deviation for a ten-ounce order of fries is at most 1.5 oz., but the manager thinks that it may be higher. He randomly weighs 49 orders of fries, which yields a mean of 11 oz. and a standard deviation of two oz.
(a) at the 5% significance level
(b) at the 1% significance level
Weights in selected apple batch (in grams): 158; 167; 149; 169; 164; 139; 154; 150; 157; 171; 152; 161; 141; 166; 172;