7.1 Inference for Two Dependent Samples (Matched Pairs)

1. Seven eighth graders at Kennedy Middle School measured how far (measured in feet) they could throw the shot put with their dominant (writing) hand and their weaker (non-writing) hand. They thought that they could throw equal distances with either hand. The data were collected and recorded below.

Figure 7.9

Conduct a hypothesis test to determine whether the mean difference in distances between the children’s dominant and weaker hands is significant.

2. Five ball players think they can throw the same distance with their dominant hand (throwing) and off-hand (catching hand). The data were collected and recorded below. Conduct a hypothesis test to determine whether the mean difference in distances between the dominant and off-hand is significant. Test at the 5% level.

Figure 7.10

3. A study was conducted to test the effectiveness of a software patch in reducing system failures over a six-month period. Results for randomly selected installations are shown below. The “before” value is matched to an “after” value, and the differences are calculated. The differences have a normal distribution. Test at the 1% significance level.

Figure 7.11

1. What is the random variable?
2. State the null and alternative hypotheses.
3. What is the p-value?
4. Draw the graph of the p-value.
5. What conclusion can you draw about the software patch?

4. A study was conducted to test the effectiveness of a juggling class. Before the class started, six subjects juggled as many balls as they could at once. After the class, the same six subjects juggled as many balls as they could. The differences in the number of balls are calculated. The differences have a normal distribution. Test at the 1% significance level.

Figure 7.12

1. State the null and alternative hypotheses.
2. What is the p-value?
3. What is the sample mean difference?
4. Draw the graph of the p-value.
5. What conclusion can you draw about the juggling class?

5. A doctor wants to know if a blood pressure medication is effective. Six subjects have their systolic blood pressures recorded. After twelve weeks on the medication, the same six subjects have their blood pressure recorded again. Test at the 1% significance level.

Figure 7.13

1. State the null and alternative hypotheses.
2. What is the test statistic?
3. What is the p-value?
4. What is the sample mean difference?
5. What is the conclusion?

NOTE: If you are using a Student’s t-distribution, including for paired data, you may assume that the underlying population is normally distributed. (When using these tests in a real situation, you must first prove that assumption, however.)

6. Ten individuals went on a low–fat diet for 12 weeks to lower their cholesterol. The data are recorded below. Do you think that their cholesterol levels were significantly lowered?

Figure 7.14

7. A new AIDS prevention drug was tried on a group of 224 HIV-positive patients. Forty-five patients developed AIDS after four years. In a control group of 224 HIV-positive patients, 68 developed AIDS after four years. We want to test whether the method of treatment reduces the proportion of patients that develop AIDS after four years or if the proportions of the treated group and the untreated group stay the same.

Let the subscript t = treated patient and ut = untreated patient.

The appropriate hypotheses are:

1. H₀: p_t < p_ut and H_a: p_t ≥ p_ut
2. H₀: p_t ≤ p_ut and H_a: p_t > p_ut
3. H₀: p_t = p_ut and H_a: p_t ≠ p_ut
4. H₀: p_t = p_ut and H_a: p_t < p_ut

If the p-value is 0.0062 what is the conclusion? Use α = 0.05.

1. The method has no effect.
2. There is sufficient evidence to conclude that the method reduces the proportion of HIV-positive patients who develop AIDS after four years.
3. There is sufficient evidence to conclude that the method increases the proportion of HIV-positive patients who develop AIDS after four years.
4. There is insufficient evidence to conclude that the method reduces the proportion of HIV-positive patients who develop AIDS after four years.

8. An experiment is conducted to show that blood pressure can be consciously reduced in people trained in a “biofeedback exercise program.” Six subjects were randomly selected, and blood pressure measurements were recorded before and after the training. The difference between blood pressures was calculated (after – before) producing the following results: = −10.2 and s_d = 8.4. Using the data, test the hypothesis that the blood pressure has decreased after the training.

1. What is the distribution for the test?
2. If α = 0.05, what is the p-value and the conclusion?

9. A golf instructor is interested in determining if her new technique for improving players’ golf scores is effective. She records the 18-hole scores for four new students before they learned the technique and then after they took her class. She conducts a hypothesis test. The data are as follows.

Figure 7.15

The correct decision is:

1. Reject H₀.
2. Do not reject H₀.

10. A local cancer support group believes that the estimate for new female breast cancer cases in the South is higher in 2013 than in 2012. The group compared the estimates of new female breast cancer cases by southern state in 2012 and in 2013. The results are shown below.

Figure 7.16

Test: Two matched pairs or paired samples (t-test)

Random variable:

Distribution: t₁₂

H₀: μ_d = 0

H_a: μ_d > 0

The mean of the differences of new female breast cancer cases in the South between 2013 and 2012 is greater than zero. The estimate for new female breast cancer cases in the South is higher in 2013 than in 2012.

Graph: Right-tailed

p-value: 0.0004

Decision: Reject H₀.

Conclusion: At the 5% level of significance, from the sample data, there is sufficient evidence to conclude that there was a higher estimate of new female breast cancer cases in 2013 than in 2012.

11. A traveler wanted to know if the prices of hotels are different in the ten cities that he visits the most often. The list of the cities with the corresponding hotel prices for his two favorite hotel chains is shown below. Test at the 1% level of significance.

Figure 7.18

12. A politician asked his staff to determine whether the underemployment rate in the Northeast decreased from 2011 to 2012. The results are shown in Figure 7.19.

Figure 7.19

Test: Matched or paired samples (t-test)

Difference data: {–0.9, –3.7, –3.2, –0.5, 0.6, –1.9, –0.5, 0.2, 0.6, 0.4, 1.7, –2.4, 1.8}

Random variable:

Distribution:

H₀: μ_d = 0

H_a: μ_d < 0

The mean of the differences of the rate of underemployment in the Northeastern states between 2012 and 2011 is less than zero. The underemployment rate went down from 2011 to 2012.

Graph: Left-tailed

p-value: 0.1207

Decision: Do not reject H₀.

Conclusion: At the 5% level of significance, from the sample data, there is not sufficient evidence to conclude that there was a decrease in the underemployment rates of the Northeastern states from 2011 to 2012.

For Questions 13–22, indicate which of the following choices best identifies the hypothesis test:

1. independent group means, population standard deviations and/or variances known
2. independent group means, population standard deviations and/or variances unknown
3. matched or paired samples
4. single mean
5. two proportions
6. single proportion

13. A powder diet is tested on 49 people, and a liquid diet is tested on 36 different people. The population standard deviations are two pounds and three pounds, respectively. Of interest is whether the liquid diet yields a higher mean weight loss than the powder diet.

14. A new chocolate bar is taste-tested on consumers. Of interest is whether the proportion of children who like the new chocolate bar is greater than the proportion of adults who like it.

15. The mean number of English courses taken in a two–year time period by male and female college students is believed to be about the same. An experiment is conducted and data are collected from 9 male students and 16 female students.

16. A football league reported that the mean number of touchdowns per game was five. A study is done to determine if the mean number of touchdowns has decreased.

17. A study is done to determine if students in the California state university system take longer to graduate than students enrolled in private universities. One hundred students from both the California state university system and private universities are surveyed. From years of research, it is known that the population standard deviations are 1.5811 years and one year, respectively.

18. According to a YWCA Rape Crisis Center newsletter, 75% of rape victims know their attackers. A study is done to verify this.

19. According to a recent study, US companies have a mean maternity leave of six weeks.

20. A recent drug survey showed an increase in the use of drugs and alcohol among local high school students as compared to the national percent. Suppose that a survey of 100 local youths and 100 national youths is conducted to see if the proportion of drug and alcohol use is higher locally than nationally.

21. A new SAT study course is tested on 12 individuals. Pre-course and post-course scores are recorded. Of interest is the mean increase in SAT scores. The following data are collected:

Figure 7.21

22. University of Michigan researchers reported in the Journal of the National Cancer Institute that quitting smoking is especially beneficial for those under age 49. In this American Cancer Society study, the risk (probability) of dying of lung cancer was about the same as for those who had never smoked.^[1]

23. Lesley E. Tan investigated the relationship between left-handedness vs. right-handedness and motor competence in preschool children. Random samples of 41 left-handed preschool children and 41 right-handed preschool children were given several tests of motor skills to determine if there is evidence of a difference between the children based on this experiment. The experiment produced the means and standard deviations shown below. Determine the appropriate test and best distribution to use for that test.

Figure 7.22

1. Two independent means, normal distribution
2. Two independent means, Student’s t-distribution
3. Matched or paired samples, Student’s t-distribution
4. Two population proportions, normal distribution

24. A golf instructor is interested in determining if her new technique for improving players’ golf scores is effective. She records the 18-hole scores of four new students before they learned the technique and then after they took her class. She conducts a hypothesis test. The data are:

Figure 7.23

This is:

1. a test of two independent means.
2. a test of two proportions.
3. a test of a single mean.
4. a test of a single proportion.

7.2 Inference for Two Independent Sample Means

1. The mean lasting time of two competing floor waxes is to be compared. Twenty floors are randomly assigned to test each wax. Both populations have a normal distributions. The data are recorded below. Does the data indicate that Wax 1 is more effective than Wax 2? Test at a 5% level of significance.

Figure 7.24

2. The means of the number of revolutions per minute of two competing engines are to be compared. Thirty engines are randomly assigned to be tested. Both populations have normal distributions. The data are recorded below. Do the data indicate that Engine 2 has higher RPM than Engine 1? Test at a 5% level of significance.

Figure 7.25

3. The mean speeds of fastball pitches from two different baseball pitchers are to be compared. A sample of 14 fastball pitches is measured from each pitcher. The populations have normal distributions. The data are recorded below. Scouters believe that Rodriguez pitches a speedier fastball.

Figure 7.26

1. What is the random variable?
2. State the null and alternative hypotheses.
3. What is the test statistic?
4. What is the p-value?
5. At the 1% significance level, what is your conclusion?

4. A researcher is testing the effects of plant food on plant growth. Nine plants have been given the plant food. Another nine plants have not been given the plant food. The heights of the plants are recorded after eight weeks. The populations have normal distributions. The following table is the result. The researcher thinks the food makes the plants grow taller.

Figure 7.27

1. Is the population standard deviation known or unknown?
2. State the null and alternative hypotheses.
3. What is the p-value?
4. Draw the graph of the p-value.
5. At the 1% significance level, what is your conclusion?

5. Two metal alloys are being considered as material for ball bearings. The mean melting point of the two alloys is to be compared. Fifteen pieces of each metal are being tested. Both populations have normal distributions. The following table is the result. It is believed that Alloy Zeta has a different melting point.

Figure 7.28

1. State the null and alternative hypotheses.
2. Is this a right-, left-, or two-tailed test?
3. What is the p-value?
4. Draw the graph of the p-value.
5. At the 1% significance level, what is your conclusion?

6. Parents of teenage boys often complain that auto insurance costs more, on average, for teenage boys than for teenage girls. A group of concerned parents examines a random sample of insurance bills. The mean annual cost for 36 teenage boys was $679. For 23 teenage girls, it was $559. From past years, it is known that the population standard deviation for each group is $180. Determine whether or not you believe that the mean cost for auto insurance for teenage boys is greater than that for teenage girls.

Subscripts: 1 = boys, 2 = girls

1. State the null and alternative hypotheses.
2. What is the random variable?
3. Which distribution should you use for this problem?
4. What is the test statistic?
5. What is the p-value?
6. At the 5% significance level, what is your decision and conclusion?

7. A group of transfer bound students wondered if they will spend the same mean amount on texts and supplies each year at their four-year university as they have at their community college. They conducted a random survey of 54 students at their community college and 66 students at their local four-year university. The sample means were $947 and $1,011, respectively. The population standard deviations are known to be $254 and $87, respectively. Conduct a hypothesis test to determine if the means are statistically the same.

8. Some manufacturers claim that non-hybrid sedan cars have a lower mean miles per gallon (mpg) than hybrid ones. Suppose that consumers test 21 hybrid sedans and get a mean of 31 mpg with a standard deviation of seven mpg. Thirty-one non-hybrid sedans get a mean of 22 mpg with a standard deviation of four mpg. Suppose that the population standard deviations are known to be six and three, respectively. Conduct a hypothesis test to evaluate the manufacturers claim.

Subscripts: 1 = non-hybrid sedans, 2 = hybrid sedans

1. State the null and alternative hypotheses.
2. What is the random variable?
3. Which distribution should you use for this problem?
4. What is the test statistic?
5. What is the p-value?
6. At the 5% significance level, what is your decision and conclusion?

9. A baseball fan wanted to know if there is a difference between the number of games played in a World Series when the American League won the series versus when the National League won the series. From 1922 to 2012, the population standard deviation of games won by the American League was 1.14, and the population standard deviation of games won by the National League was 1.11. Of 19 randomly selected World Series games won by the American League, the mean number of games won was 5.76. The mean number of 17 randomly selected games won by the National League was 5.42. Conduct a hypothesis test.

10. One of the questions in a study of marital satisfaction of dual-career couples was to rate the statement “I’m pleased with the way we divide the responsibilities for childcare.” The ratings went from one (strongly agree) to five (strongly disagree). The table below contains ten of the paired responses for husbands and wives. Conduct a hypothesis test to see if the mean difference in the husband’s versus the wife’s satisfaction level is negative (meaning that, within the partnership, the husband is happier than the wife).

Figure 7.29

11. The average amount of time boys and girls aged seven to 11 spend playing sports each day is believed to be the same. A study is done and data are collected, resulting in the data below. Each populations has a normal distribution.

Figure 7.30

Is there a difference in the mean amount of time boys and girls aged seven to 11 play sports each day? Test at the 5% level of significance.

12. Two samples are shown in the table below. Both have normal distributions. The means for the two populations are thought to be the same. Is there a difference in the means? Test at the 5% level of significance.

Figure 7.31

NOTE: When the sum of the sample sizes is larger than 30 (n₁ + n₂ > 30), you can use the normal distribution to approximate the Student’s t.

13. A study is done by a community group in two neighboring colleges to determine which one graduates students with more math classes. College A samples 11 graduates. Their average is four math classes with a standard deviation of 1.5 math classes. College B samples nine graduates. Their average is 3.5 math classes with a standard deviation of one math class. The community group believes that a student who graduates from College A has taken more math classes, on the average. Both populations have a normal distribution. Test at a 1% significance level. Answer the following questions.

1. Is this a test of two means or two proportions?
2. Are the populations standard deviations known or unknown?
3. Which distribution do you use to perform the test?
4. What is the random variable?
5. What are the null and alternate hypotheses? Write the null and alternate hypotheses in words and in symbols.
6. Is this test right-, left-, or two-tailed?
7. What is the p-value?
8. Do you reject or not reject the null hypothesis?
9. What is the conclusion?

14. A study is done to determine if Company A retains its workers longer than Company B. Company A samples 15 workers, and their average time with the company is five years with a standard deviation of 1.2. Company B samples 20 workers, and their average time with the company is 4.5 years with a standard deviation of 0.8. The populations are normally distributed.

1. Are the population standard deviations known?
2. Conduct an appropriate hypothesis test. At the 5% significance level, what is your conclusion?

15. A professor at a large community college wanted to determine whether there is a difference in the means of final exam scores between students who took his statistics course online and the students who took his face-to-face statistics class. He believed that the mean of the final exam scores for the online class would be lower than that of the face-to-face class. Was the professor correct? The randomly selected 30 final exam scores from each group are listed below.

Online class: 67.6, 41.2, 85.3, 55.9, 82.4, 91.2, 73.5, 94.1, 64.7, 64.7, 70.6, 38.2, 61.8, 88.2, 70.6, 58.8, 91.2, 73.5, 82.4, 35.5, 94.1, 88.2, 64.7, 55.9, 88.2, 97.1, 85.3, 61.8, 79.4, 79.4

Face-to-face class: 77.9, 95.3, 81.2, 74.1, 98.8, 88.2, 85.9, 92.9, 87.1, 88.2, 69.4, 57.6, 69.4, 67.1, 97.6, 85.9, 88.2, 91.8, 78.8, 71.8, 98.8, 61.2, 92.9, 90.6, 97.6, 100, 95.3, 83.5, 92.9, 89.4

Is the mean of the final exam scores of the online class lower than the mean of the final exam scores of the face-to-face class? Test at a 5% significance level. Answer the following questions:

1. Is this a test of two means or two proportions?
2. Are the population standard deviations known or unknown?
3. Which distribution do you use to perform the test?
4. What is the random variable?
5. What are the null and alternative hypotheses? Write the null and alternative hypotheses in words and in symbols.
6. Is this test right, left, or two tailed?
7. What is the p-value?
8. Do you reject or not reject the null hypothesis?
9. At the            level of significance, from the sample data, there           (is/is not) sufficient evidence to conclude that          .

16. Cohen’s d is a measure of effect size based on the differences between two means. Cohen’s d, named for United States statistician Jacob Cohen, measures the relative strength of the differences between the means of two populations based on sample data. The calculated value of effect size is then compared to Cohen’s standards of small, medium, and large effect sizes.

Figure 7.32

Cohen’s d is the measure of the difference between two means divided by the pooled standard deviation,

d =

where

.

Calculate Cohen’s d for Question 14. Is the size of the effect small, medium, or large? Explain what the size of the effect means for this problem.

17. Weighted alpha is a measure of risk-adjusted performance of stocks over a period of a year. A high positive weighted alpha signifies a stock whose price has risen, while a small positive weighted alpha indicates an unchanged stock price during the time period. Weighted alpha is used to identify companies with strong upward or downward trends. The weighted alpha for the top 30 stocks of banks in the Northeast and in the West as identified by Nasdaq on May 24, 2013, are listed in the tables below.

Northeast: 94.2, 75.2, 69.6, 52.0, 48.0, 41.9, 36.4, 33.4, 31.5, 27.6, 77.3, 71.9, 67.5, 50.6, 46.2, 38.4, 35.2, 33.0, 28.7, 26.5, 76.3, 71.7, 56.3, 48.7, 43.2, 37.6, 33.7, 31.8, 28.5, 26.0

West: 126.0, 70.6, 65.2, 51.4, 45.5, 37.0, 33.0, 29.6, 23.7, 22.6, 116.1, 70.6, 58.2, 51.2, 43.2, 36.0, 31.4, 28.7, 23.5, 21.6, 78.2, 68.2, 55.6, 50.3, 39.0, 34.1, 31.0, 25.3, 23.4, 21.5

Is there a difference in the weighted alpha of the top 30 stocks of banks in the Northeast and in the West? Test at a 5% significance level. Answer the following questions:

1. Is this a test of two means or two proportions?
2. Are the population standard deviations known or unknown?
3. Which distribution do you use to perform the test?
4. What is the random variable?
5. What are the null and alternative hypotheses? Write the null and alternative hypotheses in words and in symbols.
6. Is this test right-, left-, or two-tailed?
7. What is the p-value?
8. Do you reject or not reject the null hypothesis?
9. At the         level of significance, from the sample data, there          (is/is not) sufficient evidence to conclude that        .
10. Calculate Cohen’s d and interpret it.

For the next 15 exercises, indicate whether the hypothesis test is for:

1. independent group means, population standard deviations, and/or variances known
2. independent group means, population standard deviations, and/or variances unknown
3. matched or paired samples
4. single mean
5. two proportions
6. single proportion

18. It is believed that 70% of males pass their drivers test in the first attempt, while 65% of females pass the test in the first attempt. Of interest is whether the proportions are in fact equal.

19. A new laundry detergent is tested on consumers. Of interest is the proportion of consumers who prefer the new brand over the leading competitor. A study is done to test this.

20. A new windshield treatment claims to repel water more effectively. Ten windshields are tested by simulating rain without the new treatment. The same windshields are then treated, and the experiment is run again. A hypothesis test is conducted.

21. The known standard deviation in salary for all mid-level professionals in the financial industry is $11,000. Company A and Company B are in the financial industry. Suppose samples are taken of mid-level professionals from Company A and from Company B. The sample mean salary for mid-level professionals in Company A is $80,000. The sample mean salary for mid-level professionals in Company B is $96,000. Company A and Company B management want to know if their mid-level professionals are paid differently, on average.

22. The average worker in Germany gets eight weeks of paid vacation.

23. According to a television commercial, 80% of dentists agree that Ultrafresh toothpaste is the best on the market.

24. It is believed that the average grade on an English essay in a particular school system for females is higher than for males. A random sample of 31 females had a mean score of 82 with a standard deviation of three, and a random sample of 25 males had a mean score of 76 with a standard deviation of four.

25. The league mean batting average is 0.280 with a known standard deviation of 0.06. The Rattlers and the Vikings belong to the league. The mean batting average for a sample of eight Rattlers is 0.210, and the mean batting average for a sample of eight Vikings is 0.260. There are 24 players on the Rattlers and 19 players on the Vikings. Are the batting averages of the Rattlers and Vikings statistically different?

26. In a random sample of 100 forests in the United States, 56 were coniferous or contained conifers. In a random sample of 80 forests in Mexico, 40 were coniferous or contained conifers. Is the proportion of conifers in the United States statistically more than the proportion of conifers in Mexico?

27. A new medicine is said to help improve sleep. Eight subjects are picked at random and given the medicine. The means hours slept for each person were recorded before starting the medication and after.

28. It is thought that teenagers sleep more than adults on average. A study is done to verify this. A sample of 16 teenagers has a mean of 8.9 hours slept and a standard deviation of 1.2. A sample of 12 adults has a mean of 6.9 hours slept and a standard deviation of 0.6.

29. Varsity athletes practice five times a week, on average.

30. A sample of 12 in-state graduate school programs at School A has a mean tuition of $64,000 with a standard deviation of $8,000. At School B, a sample of 16 in-state graduate programs has a mean of $80,000 with a standard deviation of $6,000. On average, are the mean tuitions different?

31. A new WiFi range booster is being offered to consumers. A researcher tests the native range of 12 different routers under the same conditions. The ranges are recorded. Then the researcher uses the new WiFi range booster and records the new ranges. Does the new WiFi range booster do a better job?

32. A high school principal claims that 30% of student athletes drive themselves to school, while 4% of non-athletes drive themselves to school. In a sample of 20 student athletes, 45% drive themselves to school. In a sample of 35 non-athlete students, 6% drive themselves to school. Is the percent of student athletes who drive themselves to school more than the percent of nonathletes?

33. A study is done to determine which of two soft drinks has more sugar. There are 13 cans of Beverage A in a sample and six cans of Beverage B. The mean amount of sugar in Beverage A is 36 grams with a standard deviation of 0.6 grams. The mean amount of sugar in Beverage B is 38 grams with a standard deviation of 0.8 grams. The researchers believe that Beverage B has more sugar than Beverage A, on average. Both populations have normal distributions.

1. Are standard deviations known or unknown?
2. What is the random variable?
3. The random variable is the difference between the mean amounts of sugar in the two soft drinks.
4. Is this a one-tailed or two-tailed test?

34. The US Center for Disease Control reports that the mean life expectancy was 47.6 years for White people born in 1900 and 33.0 years for non-White people. Suppose that you randomly survey death records for people born in 1900 in a certain county. Of the 124 White people, the mean life span was 45.3 years with a standard deviation of 12.7 years. Of the 82 non-White people, the mean life span was 34.1 years with a standard deviation of 15.6 years. Conduct a hypothesis test to see if the mean life spans in the county were the same for White people and non-White people.

1. Is this a test of means or proportions?
2. State the null and alternative hypotheses.
3. Is this a right-tailed, left-tailed, or two-tailed test?
4. In symbols, what is the random variable of interest for this test?
5. In words, define the random variable of interest for this test.
6. Which distribution (normal or Student’s t) would you use for this hypothesis test?
7. Explain why you chose the distribution you did.
8. Calculate the test statistic and p-value.
9. Sketch a graph of the situation. Label the horizontal axis. Mark the hypothesized difference and the sample difference. Shade the area corresponding to the p-value.
10. Find the p-value.
11. At a pre-conceived α = 0.05, what is your:
    • Decision
    • Reason for the decision
    • Conclusion (write out in a complete sentence)
12. Does it appear that the means are the same? Why or why not?

NOTE: If you are using a Student’s t-distribution, including for paired data, you may assume that the underlying population is normally distributed. (When using these tests in a real situation, you must first prove that assumption, however.)

35. The mean number of English courses taken in a two–year time period by male and female college students is believed to be about the same. An experiment is conducted, and data are collected from 29 males and 16 females. The males took an average of three English courses with a standard deviation of 0.8. The females took an average of four English courses with a standard deviation of 1.0. Are the means statistically the same?

36. A student at a four-year college claims that mean enrollment at four–year colleges is higher than at two–year colleges in the United States. Two surveys are conducted. Of the 35 two–year colleges surveyed, the mean enrollment was 5,068 with a standard deviation of 4,777. Of the 35 four-year colleges surveyed, the mean enrollment was 5,466 with a standard deviation of 8,191.

Subscripts: 1: two-year colleges, 2: four-year colleges

37. At Rachel’s 11th birthday party, eight girls were timed to see how long (in seconds) they could hold their breath in a relaxed position. After a two-minute rest, they timed themselves while jumping. The girls thought that the mean difference between their jumping and relaxed times would be zero. Test their hypothesis.

Figure 7.33

38. Mean entry-level salaries for college graduates with mechanical engineering degrees and electrical engineering degrees are believed to be approximately the same. A recruiting office thinks that the mean mechanical engineering salary is actually lower than the mean electrical engineering salary. The recruiting office randomly surveys 50 entry-level mechanical engineers and 60 entry-level electrical engineers. Their mean salaries were $46,100 and $46,700, respectively. Their standard deviations were $3,450 and $4,210, respectively. Conduct a hypothesis test to determine if you agree that the mean entry-level mechanical engineering salary is lower than the mean entry-level electrical engineering salary.

Subscripts: 1: mechanical engineering, 2: electrical engineering

39. Marketing companies have collected data implying that teenage girls use more ring tones on their cellular phones than teenage boys do. In one particular study of 40 randomly chosen teenage girls and boys (20 of each) with cellular phones, the mean number of ring tones for the girls was 3.2 with a standard deviation of 1.5. The mean for the boys was 1.7 with a standard deviation of 0.8. Conduct a hypothesis test to determine if the means are approximately the same or if the girls’ mean is higher than the boys’ mean.

40. The Eastern and Western Major League Soccer conferences have a new Reserve Division that allows new players to develop their skills. Data for a randomly picked date showed the following annual goals. Conduct a hypothesis test. What is the exact distribution for the hypothesis test? If the level of significance is 0.05, what is the conclusion?

Figure 7.34

41. Suppose a statistics instructor believes that there is no significant difference between the mean class scores of statistics day students on Exam 2 and statistics night students on Exam 2. She takes random samples from each of the populations. The mean and standard deviation for 35 statistics day students were 75.86 and 16.91. The mean and standard deviation for 37 statistics night students were 75.41 and 19.73. A concluding statement is:

1. There is sufficient evidence to conclude that the statistics night students’ mean on Exam 2 is better than the statistics day students’ mean on Exam 2.
2. There is insufficient evidence to conclude that the statistics day students’ mean on Exam 2 is better than the statistics night students’ mean on Exam 2.
3. There is insufficient evidence to conclude that there is a significant difference between the means of the statistics day students and night students on Exam 2.
4. There is sufficient evidence to conclude that there is a significant difference between the means of the statistics day students and night students on Exam 2.

42. Researchers interviewed street sex workers in Canada and the United States. The mean age of the 100 Canadian sex workers upon entering sex work was 18 with a standard deviation of six. The mean age of the 130 United States sex workers upon entering their work was 20 with a standard deviation of eight. Is the mean age of entering sex work in Canada lower than the mean age in the United States? Test at a 1% significance level.

43. A powder diet is tested on 49 people, and a liquid diet is tested on 36 different people. Of interest is whether the liquid diet yields a higher mean weight loss than the powder diet. The powder diet group had a mean weight loss of 42 pounds with a standard deviation of 12 pounds. The liquid diet group had a mean weight loss of 45 pounds with a standard deviation of 14 pounds. Conduct a hypothesis test.

44. Suppose a statistics instructor believes that there is no significant difference between the mean class scores of statistics day students on Exam 2 and statistics night students on Exam 2. She takes random samples from each of the populations. The mean and standard deviation for 35 statistics day students were 75.86 and 16.91, respectively. The mean and standard deviation for 37 statistics night students were 75.41 and 19.73. The “day” subscript refers to the statistics day students. The “night” subscript refers to the statistics night students. An appropriate alternative hypothesis for the hypothesis test is:

1. μ_day > μ_night
2. μ_day < μ_night
3. μ_day = μ_night
4. μ_day ≠ μ_night

7.3 Inference for Two-Sample Proportions

1. A research study was conducted about gender differences in “sexting.” The researcher believed that the proportion of girls involved in sexting is less than the proportion of boys involved. The data collected in the spring of 2010 among a random sample of middle and high school students in a large school district in the southern United States is summarized below. Is the proportion of girls sending sexts less than the proportion of boys sexting? Test at a 1% level of significance.^[2]

Figure 7.35

3. An interested citizen wanted to know if Democratic US senators are older than Republican US senators, on average. On May 26 2013, the mean age of 30 randomly selected Republican Senators was 61 years 247 days old (61.675 years) with a standard deviation of 10.17 years. The mean age of 30 randomly selected Democratic senators was 61 years 257 days old (61.704 years) with a standard deviation of 9.55 years.^[4]

Do the data indicate that Democratic senators are older than Republican senators, on average? Test at a 5% level of significance.

4. A concerned group of citizens wanted to know if the proportion of forcible rapes in Texas was different in 2011 than in 2010. Their research showed that, of the 113,231 violent crimes in Texas in 2010, 7,622 of them were forcible rapes. In 2011, 7,439 of the 104,873 violent crimes were in the forcible rape category.^[5] Test at a 5% significance level. Answer the following questions:

1. Is this a test of two means or two proportions?
2. Which distribution do you use to perform the test?
3. What is the random variable?
4. What are the null and alternative hypothesis? Write the null and alternative hypothesis in symbols.
5. Is this test right-, left-, or two-tailed?
6. What is the p-value?
7. Do you reject or not reject the null hypothesis?
8. At the          level of significance, from the sample data, there         (is/is not) sufficient evidence to conclude that        .

5. Two types of phone operating system are being tested to determine if there is a difference in the proportions of system failures (crashes). Fifteen out of a random sample of 150 phones with OS₁ had system failures within the first eight hours of operation. Nine out of another random sample of 150 phones with OS₂ had system failures within the first eight hours of operation. OS₂ is believed to be more stable (have fewer crashes) than OS₁.

1. Is this a test of means or proportions?
2. What is the random variable?
3. State the null and alternative hypotheses.
4. What is the p-value?
5. What can you conclude about the two operating systems?

6. In the recent Census, three percent of the US population reported being of two or more races. However, the percent varies tremendously from state to state. Suppose that two random surveys are conducted. In the first random survey, out of 1,000 North Dakotans, only nine people reported being of two or more races. In the second random survey, out of 500 Nevadans, 17 people reported being of two or more races.^[6] Conduct a hypothesis test to determine if the population percents are the same for the two states or if the percent for Nevada is statistically higher than for North Dakota.

1. Is this a test of means or proportions?
2. State the null and alternative hypotheses.
3. Is this a right-tailed, left-tailed, or two-tailed test? How do you know?
4. What is the random variable of interest for this test?
5. In words, define the random variable for this test.
6. Which distribution (normal or Student’s t) would you use for this hypothesis test?
7. Explain why you chose the distribution you did.
8. Calculate the test statistic.
9. Sketch a graph of the situation. Mark the hypothesized difference and the sample difference. Shade the area corresponding to the p-value.
10. Find the p-value.
11. At a pre-conceived α = 0.05, what is your:
    • Decision
    • Reason for the decision
    • Conclusion (write out in a complete sentence)
12. Does it appear that the proportion of Nevadans who are two or more races is higher than the proportion of North Dakotans? Why or why not?

9. We are interested in whether the proportions of female suicide victims for ages 15 to 24 are the same for the White and Black races in the United States. We randomly pick one year, 1992, to compare the races. The number of suicides estimated in the United States in 1992 for White females is 4,930, of whom 580 were aged 15 to 24. The estimate for Black females is 330, of whom 40 were aged 15 to 24.^[7] We will let female suicide victims be our population.

10. Elizabeth Mjelde, an art history professor, was interested in whether the value from the golden ratio formula was the same in the Whitney Exhibit for works from 1900 to 1919 as for works from 1920 to 1942. Thirty-seven early works were sampled, averaging 1.74 with a standard deviation of 0.11. Sixty-five of the later works were sampled, averaging 1.746 with a standard deviation of 0.1064.^[8] Do you think that there is a significant difference in the golden ratio calculation?

11. A recent year was randomly picked from 1985 to the present. In that year, there were 2,051 Hispanic students at Cabrillo College out of a total of 12,328 students. At Lake Tahoe College, there were 321 Hispanic students out of a total of 2,441 students.^[9] In general, do you think that the percent of Hispanic students at the two colleges is basically the same or different?

Subscripts: 1 = Cabrillo College, 2 = Lake Tahoe College

12. Neuroinvasive West Nile virus is a severe disease that affects a person’s nervous system. It is spread by the Culex species of mosquito. In the United States in 2010, there were 629 reported cases of neuroinvasive West Nile virus out of a total of 1,021 reported cases, and there were 486 neuroinvasive reported cases out of a total of 712 cases reported in 2011.^[10] Is the 2011 proportion of neuroinvasive West Nile virus cases more than the 2010 proportion of neuroinvasive West Nile virus cases? Using a 1% level of significance, conduct an appropriate hypothesis test.

Subscripts: 2011 = 2011 group, 2010 = 2010 group

a. This is:

1. a test of two proportions.
2. a test of two independent means.
3. a test of a single mean.
4. a test of matched pairs.

b. An appropriate null hypothesis is:

1. p₂₀₁₁ ≤ p_2010.
2. p₂₀₁₁ ≥ p_2010.
3. μ₂₀₁₁ ≤ μ_2010.
4. p₂₀₁₁ > p_2010.

c. The p-value is 0.0022. At a 1% level of significance, the appropriate conclusion is

1. There is sufficient evidence to conclude that the proportion of people in the United States in 2011 who contracted neuroinvasive West Nile disease is less than the proportion of people in the United States in 2010 who contracted neuroinvasive West Nile disease.
2. There is insufficient evidence to conclude that the proportion of people in the United States in 2011 who contracted neuroinvasive West Nile disease is more than the proportion of people in the United States in 2010 who contracted neuroinvasive West Nile disease.
3. There is insufficient evidence to conclude that the proportion of people in the United States in 2011 who contracted neuroinvasive West Nile disease is less than the proportion of people in the United States in 2010 who contracted neuroinvasive West Nile disease.
4. There is sufficient evidence to conclude that the proportion of people in the United States in 2011 who contracted neuroinvasive West Nile disease is more than the proportion of people in the United States in 2010 who contracted neuroinvasive West Nile disease.

13. Researchers conducted a study to find out if there is a difference in the use of eReaders by different age groups. Randomly selected participants were divided into two age groups. In the 16- to 29-year-old group, 7% of the 628 surveyed use eReaders, while 11% of the 2,309 participants 30 years old and older use eReaders.^[11]

14. Adults aged 18 years old and older were randomly selected for a survey on obesity. Adults are considered obese if their body mass index (BMI) is at least 30. The researchers wanted to determine if the proportion of women who are obese in the South is less than the proportion of southern men who are obese. The results are shown below.^[12] Test at the 1% level of significance.

Figure 7.36

15. Two computer users were discussing tablet computers. At one point in time, a higher proportion of people ages 16 to 29 use tablets than the proportion of people age 30 and older. The figure below details the number of tablet owners for each age group. Test at the 1% level of significance.

Figure 7.37

16. A group of friends debated whether more men use smartphones than women. They consulted a research study of smartphone use among adults. The results of the survey indicate that, of the 973 men randomly sampled, 379 use smartphones. For women, 404 of the 1,304 who were randomly sampled use smartphones.^[13] Test at the 5% level of significance.

17. While her husband spent 2.5 hours picking out new speakers, a statistician decided to determine whether the percent of men who enjoy shopping for electronic equipment is higher than the percent of women who enjoy shopping for electronic equipment. The population was Saturday afternoon shoppers. Out of 67 men, 24 said they enjoyed the activity. Eight of the 24 women surveyed claimed to enjoy the activity. Interpret the results of the survey.

Subscripts: 1: men, 2: women

18. We are interested in whether children’s educational computer software costs less, on average, than children’s entertainment software. Thirty-six educational software titles were randomly picked from a catalog. The mean cost was $31.14 with a standard deviation of $4.69. Thirty-five entertainment software titles were randomly picked from the same catalog. The mean cost was $33.86 with a standard deviation of $10.87. Decide whether children’s educational software costs less, on average, than children’s entertainment software.

19. Joan Nguyen recently claimed that the proportion of college-age males with at least one pierced ear is as high as the proportion of college-age females. She conducted a survey in her classes. Out of 107 males, 20 had at least one pierced ear. Out of 92 females, 47 had at least one pierced ear. Do you believe that the proportion of males has reached the proportion of females?

20. “To Breakfast or Not to Breakfast?” by Richard Ayore:

In the American society, birthdays are one of those days that everyone looks forward to. People of different ages and peer groups gather to mark the 18th, 20th, …, birthdays. During this time, one looks back to see what he or she has achieved for the past year and also focuses ahead for more to come.

If, by any chance, I am invited to one of these parties, my experience is always different. Instead of dancing around with my friends while the music is booming, I get carried away by memories of my family back home in Kenya. I remember the good times I had with my brothers and sister while we did our daily routine.

Every morning, I remember we went to the shamba (garden) to weed our crops. I remember one day arguing with my brother as to why he always remained behind just to join us an hour later. In his defense, he said that he preferred waiting for breakfast before he came to weed. He said, “This is why I always work more hours than you guys!”

And so, to prove him wrong or right, we decided to give it a try. One day, we went to work as usual without breakfast and recorded the time we could work before getting tired and stopping. On the next day, we all ate breakfast before going to work. We recorded how long we worked again before getting tired and stopping. Of interest was our mean increase in work time. Though not sure, my brother insisted that it was more than two hours. Using the data below, solve our problem.

Figure 7.38

Figure Descriptions

Figure 7.17: This is a normal distribution curve with mean equal to zero. A vertical line near the tail of the curve to the right of zero extends from the axis to the curve. The region under the curve to the right of the line is shaded representing p-value = 0.0004.

Figure 7.20: This is a normal distribution curve with mean equal to zero. A vertical line near the tail of the curve to the right of zero extends from the axis to the curve. The region under the curve to the right of the line is shaded representing p-value = 0.1207.

References

Figures

Figure 7.17: Figure 10.22 from OpenStax Statistics (2020) (CC BY 4.0). Retrieved from https://openstax.org/books/statistics/pages/10-solutions#fs-idm5499280-solution

Figure 7.20: Figure 10.23 from OpenStax Statistics (2020) (CC BY 4.0). Retrieved from https://openstax.org/books/statistics/pages/10-solutions#fs-idm5499280-solution

Text

Data from Educational Resources, December catalog.

Data from Hilton Hotels. Available online at http://www.hilton.com (accessed June 17, 2013).

Data from Hyatt Hotels. Available online at http://hyatt.com (accessed June 17, 2013).

Data from Statistics, United States Department of Health and Human Services.

Data from Whitney Exhibit on loan to San Jose Museum of Art.

Data from the American Cancer Society. Available online at http://www.cancer.org/index (accessed June 17, 2013).

Data from the Chancellor’s Office, California Community Colleges, November 1994.

“State of the States,” Gallup, 2013. Available online at http://www.gallup.com/poll/125066/State-States.aspx?ref=interactive (accessed June 17, 2013).

“West Nile Virus,” Centers for Disease Control and Prevention. Available online at http://www.cdc.gov/ncidod/dvbid/westnile/index.htm (accessed June 17, 2013).