Chapter 10
Statistical Inference About Means and Proportions with Two Populations
Learning Objectives
1. Be able to develop interval estimates and conduct hypothesis tests about the difference between two population means when_{ }and_{ }are known.
2. Know the properties of the sampling distribution of _{ }.
3. Be able to use the t distribution to conduct statistical inferences about the difference between two population means when_{ }and_{ }are unknown.
4. Learn how to analyze the difference between two population means when the samples are independent and when the samples are matched.
5. Be able to develop interval estimates and conduct hypothesis tests about the difference between two population proportions.
6. Know the properties of the sampling distribution of _{ }.
Solutions:
1. a. _{ } = 13.6  11.6 = 2
b. _{ }1.645
_{ }
_{ }
2 .98 (1.02 to 2.98)
c. _{ }1.96
_{ }
2 1.17 (.83 to 3.17)
2. a. _{ }
b. pvalue = 1.0000  .9788 = .0212
c. pvalue _{ } .05, reject H_{0}.
3. a. _{ }
b. pvalue = 2(.0630) = .1260
c. pvalue > .05, do not reject H_{0}.
4. a. _{ }= Population mean for smaller cruise ships
_{ }= Population mean for larger cruise ships
_{ }= 85.36 – 81.40 = 3.96
b. _{ }
_{ }
c. 3.96 ± 1.88 (2.08 to 5.84)
5. a. _{ }= 135.67 – 68.64 = 67.03
b. _{ }
c. 67.03 17.08 (49.95 to 84.11) We estimate that men spend $67.03 more
than women on Valentine’s Day with a margin of error of $17.08.
6. _{ }= Mean hotel price in Atlanta
_{ }= Mean hotel price in Houston
H_{0}: _{ }
H_{a}: _{ }
_{ }
pvalue = .0351
pvalue _{ } .05; reject H_{0}. The mean price of a hotel room in Atlanta is lower than the mean price of a hotel room in Houston.
7. a. _{ }= Population mean 2002
_{ }= Population mean 2003
H_{0}: _{ }
H_{a}: _{ }
b. With time in minutes,_{ }= 172  166 = 6 minutes
c. _{ }
pvalue = 1.0000  .9955 = .0045
pvalue _{ }.05; reject H_{0}. The population mean duration of games in 2003 is less than the population mean in 2002.
d. _{ }
_{ }
6 4.5 (1.5 to 10.5)
e. Percentage reduction: 6/172 = 3.5%. Management should be encouraged by the fact that steps taken in 2003 reduced the population mean duration of baseball games. However, the statistical analysis shows that the reduction in the mean duration is only 3.5%. The interval estimate shows the reduction in the population mean is 1.5 minutes (.9%) to 10.5 minutes (6.1%). Additional data collected by the end of the 2003 season would provide a more precise estimate. In any case, most likely the issue will continue in future years. It is expected that major league baseball would prefer that additional steps be taken to further reduce the mean duration of games.
8. a. This is an upper tail hypothesis test.
_{ }
_{ }_{ }
_{ }
pvalue = area in upper tail at z = 2.74
pvalue = 1.0000  .9969 = .0031
Since .0031 _{ } α = .05, we reject the null hypothesis. The difference is significant. We can conclude that customer service has improved for Rite Aid.
b. This is another upper tail test but it only involves one population.
_{ }
_{ }_{ }
_{ }
pvalue = area in upper tail at z = .39
pvalue = 1.0000  .6517 = .3483
Since .3483 > α = .05, we cannot reject the null hypothesis. The difference is not statistically significant.
c. This is an upper tail test similar to the one in part (a).
_{ }
_{ }_{ }
_{ }
pvalue = area in upper tail at z = 1.83
pvalue = 1.0000  .9664 = .0336
Since .0336 _{ } α = .05, we reject the null hypothesis. The difference is significant. We can conclude that customer service has improved for Expedia.
d. We will reject the null hypothesis of “no increase” if the pvalue ≤ .05. For an upper tail hypothesis test, the pvalue is the area in the upper tail at the value of the test statistic. A value of z = 1.645 provides an upper tail area of .05. So, we must solve the following equation for _{ }.
_{ }
_{ }
This tells us that as long as the 2008 score for a company exceeds the 2007 score by 1.80 or more the difference will be statistically significant.
e. The increase from 2007 to 2008 for J.C. Penney is not statistically significant because it is less than 1.80. We cannot conclude that customer service has improved for J.C. Penney.
9. a. _{ }= 22.5  20.1 = 2.4
b. _{ }
Use df = 45.
c. t_{.025} = 2.014
_{ }
d. 2.4 2.1 (.3 to 4.5)
10. a. _{ }
b. _{ }
Use df = 65
c. Using t table, area in tail is between .01 and .025
^{ }twotail pvalue is between .02 and .05.
Exact pvalue corresponding to t = 2.18 is .0329
d. pvalue_{ } .05, reject H_{0}.
11. a. _{ } _{ }
b. _{ }
_{ }
c. _{ }= 9  7 = 2
d. _{ }
Use df = 9, t_{.05} = 1.833
_{ }
2 2.17 (.17 to 4.17)
12. a. _{ }= 22.5  18.6 = 3.9
b. _{ }
Use df = 87, t_{.025} = 1.988
_{ }
3.9 (.6 to 7.2)
13. a. _{ }
_{ }
_{ }
_{ }
b. _{ }= 9.3  4.2 = 5.1 tons
Memphis is the higher volume airport and handled an average of 5.1 tons per day more than Louisville. Memphis handles more than twice the volume of Louisville.
c. _{ }
Use df = 17, t_{.025} = 2.110
_{ }
_{ }
5.1 1.82 (3.28 to 6.92)
14. a. H_{0}: _{ }
H_{a}: _{ }
b. _{ }
c. _{ }= 87.55
Rounding down, we will use a t distribution with 87 degrees of freedom. From the t table we see that t = 2.41 corresponds to a pvalue between .005 and .01.
Exact pvalue corresponding to t = 2.41 is .009.
d. pvalue_{ } .05, reject H_{0}. We conclude that the salaries of staff nurses are lower in Tampa than in Dallas.
15. _{1} for 2001 season
_{2} for 1992 season
H_{0}: _{ }
H_{a}: _{ }
b. _{ }= 60  51 = 9 days
9/51(100) = 17.6% increase in number of days.
c. _{ }
_{ }
Using t table, pvalue is between .005 and .01.
Exact pvalue corresponding to t = 2.48 is .0076
pvalue_{ } .01, reject H_{0}. There is a greater mean number of days on the disabled list in 2001.
d. Management should be concerned. Players on the disabled list have increased 32% and time on the list has increased by 17.6%. Both the increase in inquiries to players and the cost of lost playing time need to be addressed.
16. a. _{1} = population mean verbal score parents college grads
_{2} = population mean verbal score parents high school grads
H_{0}: _{ }
H_{a}: _{ }
b. _{ }
_{ }
_{ }= 525  487 = 38 points higher if parents are college grads
c. _{ }
_{ }
_{ }
_{ }
Use df = 25
Using t table, pvalue is between .025 and .05
Exact pvalue corresponding to t = 1.80 is .0420
d. pvalue_{ } .05, reject H_{0}. Conclude higher population mean verbal scores for students whose parents are college grads.
17. a. H_{0}: _{ }
H_{a}: _{ }
b. _{ }
c. _{ }
Use df = 16
Using t table, pvalue is between .025 and .05
Exact pvalue corresponding to t = 1.99 is .0320
d. pvalue_{ } .05, reject H_{0}. The consultant with more experience has a higher population mean rating.
18. a. H_{0}: _{ }
H_{a}: _{ }
b. _{ }
_{ }
Use df = 78
Using t table, pvalue is between .01 and .025
Exact pvalue corresponding to t = 2.10 is .0195
pvalue_{ } .05, reject H_{0}. The improvement is less than the stated average of 120 points.
c. _{ }
df = 78
_{ }
75_{ }43 (32 to 118)
d. This is a wide interval. A larger sample should be used to reduce the margin of error.
19. a. 1, 2, 0, 0, 2
b. _{ }
c. _{ }
d. _{ }
df = n  1 = 4
Using t table, pvalue is between .025 and .05
Exact pvalue corresponding to t = 2.24 is .0443
Reject H_{0}; conclude _{d} > 0.
20. a. 3, 1, 3, 5, 3, 0, 1
b. _{ }
c. _{ }
d. _{ }= 2
e. With 6 degrees of freedom t_{.025} = 2.447
_{ }
2 1.93 (.07 to 3.93)
21. Difference = rating after  rating before
H_{0}: _{d} 0
H_{a}: _{d} > 0
_{ }= .625 and _{ }= 1.30
_{ }
df = n  1 = 7
Using t table, pvalue is between .10 and .20
Exact pvalue corresponding to t = 1.36 is .1080
Do not reject H_{0}; we cannot conclude that seeing the commercial improves the mean potential to purchase.
22. Let d_{i} = current qtr. per share earnings – previous quarter per share earnings
_{ }
_{ }
With df = 24, t_{.025} = 2.064
_{ }
.2064 2.064 _{ }
Confidence interval: $.21 $.11 ($.10 to $.32)
Earnings have increased. The point estimate of the increase in earnings per
share is $.21 with a margin of error of $.11.
23. a. _{1} = population mean grocery expenditures
_{2} = population mean diningout expenditures
H_{0}: _{ }
H_{a}: _{ }
b. _{ }
df = n  1 = 41
pvalue^{ }0
Conclude that there is a difference between the annual population mean expenditures for groceries and for diningout.
c. Groceries has the higher mean annual expenditure by an estimated $850.
_{ }
_{ }
850 350 (500 to 1200)
24. H_{0}: _{d}≤ 0
H_{a}: _{d}> 0
Differences 177, 21, 186, 131, 22, 212, 5, 14
_{ }
_{ }
_{ }
df = n  1 = 7
Using t table, pvalue is greater than .10
Exact pvalue corresponding to t = 1.32 is .1142
Since pvalue > .10, do not reject H_{0}. We cannot conclude that airfares from Dayton are higher than those from Louisville at a α = .05 level of significance.
25. a. H_{0}: _{d}= 0
H_{a}: _{d} 0
Use difference data: 3, 2, 4, 3, 1, 2, 1, 2, 0, 0, 1, 4, 3, 1, 1
_{ }
_{ }
_{ }
df = n  1 = 14
Using t table, area is between .01 and .025.
Twotail pvalue is between .02 and .05.
Exact pvalue corresponding to t = 2.36 is .0333
pvalue_{ } .05, reject H_{0}. Conclude that there is a difference between the population mean weekly usage for the two media.
b. _{ }hours per week for cable television.
_{ }hours per week for radio.
Radio has greater usage.
26. a. H_{0}: _{d}= 0
H_{a}: _{d} 0
Differences: 2, 1, 5, 1, 1, 0, 4, 7, 6, 1, 0, 2, 3, 7, 2, 3, 1, 2, 1, 4
_{ }
_{ }
_{ }
df = n – 1 = 19
Using t table, area in tail is between .05 and .10
Twotail pvalue must be between .10 and .20
Exact pvalue corresponding to t = 1.42 is .1718
Cannot reject H_{0}. There is no significant difference between the mean scores for the first and fourth rounds.
b. _{ }= 1.05; First round scores were lower than fourth round scores.
c. α = .05 df = 19 t = 1.729
Margin of error =_{ } = _{ }
Yes, just check to see if the 90% confidence interval includes a difference of zero. If it does, the difference is not statistically significant.
90% Confidence interval: 1.05 ± 1.28 (2.33, .23)
The interval does include 0, so the difference is not statistically significant.
27. a. Difference = Price deluxe  Price Standard
H_{0}: _{d} = 10
H_{a}: _{d} 10
_{ }= 8.86 and _{ }= 2.61
_{ }
df = n  1 = 6
Using t table, area is between .10 and .20
Twotail pvalue is between .20 and .40
Exact pvalue corresponding to t = 1.16 is .2901
Do not reject H_{0}; we cannot reject the hypothesis that a $10 price differential exists.
b. 95% Confidence interval
_{ }
_{ }
_{ }or (6.45 to 11.27)
28. a. _{ }= .48  .36 = .12
b. _{ }
_{ }
.12 .0614 (.0586 to .1814)
c. _{ }
.12 .0731 (.0469 to .1931)
29. a. _{ }
_{ }
p  value = 1.0000  .9554 = .0446
b. pvalue_{ } .05; reject H_{0}.
30. _{ }= 220/400 = .55 _{ }= 192/400 = .48
_{ }
_{ }
.07 .0691 (.0009 to .1391)
7% more executives are predicting an increase in fulltime jobs. The confidence interval shows the difference may be from 0% to 14%.
31. a. Professional Golfers: _{ }= 688/1075 = .64
Amateur Golfers: _{ }= 696/1200 = .58
Professional golfers have the better putting accuracy.
b. _{ }
Professional golfers make 6% more 6foot putts than the very best amateur golfers.
c. _{ }
_{ }
.06 .04 (.02 to .10)
The confidence interval shows that professional golfers make from 2% to 10% more 6foot putts than the best amateur golfers.
32. a. _{ }
_{ }
b. _{ } = 300/811 = .3699 37% of women would ask directions
c. _{ } = 255/750 = .3400 34% of men would ask directions
d. _{ }
_{ }
Upper tail pvalue is the area to the right of the test statistic
Using normal table with z = 1.23: pvalue = 1  .8907 = .1093
pvalue > α ; do not reject _{ }
We cannot conclude that women are more likely to ask directions.
33. Let p_{1 }= the population proportion of delayed departures at Chicago O’Hare
_{ }p_{2} = the population proportion of delayed departures at Atlanta HartsfieldJackson
a. H_{0}: p_{1}  p_{2} = 0
H_{a}: p_{1}  p_{2} ≠ 0
b. _{ } = 252/900 = .28
c. _{ } = 312/1200 = .26
d. _{ }
_{ }
pvalue = 2(1  .8461) = .3078
Do Not Reject H_{0}. We cannot conclude that there is a difference between the proportion of delayed departures at the two airports.
34. a. _{ } = 192/300 = .64
b. _{ }= 117/260 = .45
c. _{ }= .64  .45 = .19
_{ }
_{ }
.19 .0813 (.1087 to .2713)
35. a. H_{0}: p_{1}  p_{2} = 0
H_{a}: p_{1}  p_{2} 0
_{ } = 63/150 = .42
_{ } = 60/200 = .30
_{ }
_{ }
pvalue = 2(1.0000  .9901) = .0198
pvalue_{ }.05, reject H_{0}. There is a difference between the recall rates for the two commercials.
b. _{ }
_{ }
.12 .1014 (.0186 to .2214)
Commercial A has the better recall rate.
36. a. _{ } = proportion of under 30 liking the ad a lot
_{ } = proportion of 30 to 49 liking the ad a lot
H_{0}: p_{1}  p_{2} = 0
H_{a}: p_{1}  p_{2} 0
b. _{ }= 49/100 = .49
_{ }= 54/150 = .36
_{ }= .49  .36 = .13
c. _{ }
_{ }
pvalue = 2(1.0000  .9798) = .0404
pvalue_{ }.05, reject H_{0}. There is a difference between the response to the ad by the younger under 30 and the older 30 to 49 age groups.
d. There is a statistically significant difference between the population proportions for the two age groups. The stronger appeal is with the younger, under 30, age group. Miller Lite is most likely pleased and encouraged by the results of the poll. "The Miller Lite Girls" ad ranked among the top three Super Bowl ads in advertising effectiveness. In addition, 49% of the younger, under 30, group liked the ad a lot. While a response of 36% for the older age group was not bad, Miller Lite probably liked, and probably expected, the higher rating among the younger audience. Since a younger audience contains the newer beer drinkers, appealing to the younger audience could bring new customers to the Miller Lite product. The older age group may be less likely to change from their established personal favorite beer because of the commercial.
37. a. H_{0}: p_{1}  p_{2} = 0
H_{a}: p_{1}  p_{2} 0
b. _{ } = 141/523 = .2696 (27%)
_{ } = 81/477 = .1698 (17%)
c. _{ }
_{ }
pvalue^{ } 0
Reject H_{0}. There is a significant difference in the population proportions. A higher flying rate in 2003 is observed.
d. It may be that the general population is more acceptable to flying on vacation in 2003. Also, frequent flyer awards and special discount air fares in 2003 may have made 2003 flying more economical.
Note: In 1993, a round trip Newark to San Francisco was $388. In 2003, a special fare for the same trip was $238.
38. H_{0}: _{1}  _{2} = 0
H_{a}: _{1}  _{2} 0
_{ }
pvalue = 2(1.0000  .9974) = .0052
pvalue_{ } .05, reject H_{0}. A difference exists with system B having the lower mean checkout time.
39. a. _{ } Mean resale price in 2006
_{ } Mean resale price in 2009
Difference = 225,897 – 170,993 = 54,904
Using sample mean prices, the 2009 resale prices are $54,904 less than in 2006.
b. _{ }
_{ }
_{ }
Use df = 54, t_{.005} = 2.670
_{ }
_{ }
54904 32931 (21,973 to 87,835)
We are 99% confident that home prices have declined by between $21,973 and $87,835.
c. To answer this question we need to conduct a onetailed hypothesis test. No value for the level of significance (α) has been given. But, most people would agree that a pvalue _{ } .01 would justify concluding that prices have declined from 2006 to 2009.
_{ }
_{ }
For t = 4.45 and df =54, we find pvalue ^{ }0.00. Thus, we are justified in concluding that existing home prices have declined between 2006 and 2009.
40. a. H_{0}: _{1}  _{2} 0
H_{a}: _{1}  _{2} > 0
b. n_{1 }= 30 n_{2} = 30
_{ }= 16.23 _{ }= 15.70
s_{1 }= 3.52 s_{2} = 3.31
_{ }
_{ }
Use df = 57
Using t table, pvalue is greater than .20
Exact pvalue corresponding to t = .60 is .2754
pvalue > .05, do not reject H_{0}. Cannot conclude that the mutual funds with a load have a greater mean rate of return.
41. a. n_{1} = 10 n_{2} = 8
_{ }= 21.2 _{ }= 22.8
s_{1} = 2.70 s_{2} = 3.55
_{ }= 21.2  22.8 = 1.6
Kitchens are less expensive by $1600.
b. _{ }
Use df = 12, t_{.05} = 1.782
_{ }
1.6 2.7 (4.3 to 1.1)
42. a.
January 1 April 30 _{ } _{ } _{ } 10.1312.212.084.5320.520928.3325.482.850.400.160073.9766.107.875.4229.376416.3019.323.025.4729.920945.2743.052.220.230.052916.8815.461.421.031.06092.295.983.696.1437.699616.2012.653.551.101.210059.8352.367.475.0225.200431.5333.001.473.9215.366419.4420.260.823.2710.692917.7319.341.614.0616.483617.7113.364.351.903.610043.5136.187.334.8823.814461.8249.4412.389.93 98.6049Sum 36.75 313.7742
_{ } The mean price per share declined $2.45 over the four months.
b. _{ }
df = n  1 = 14, t_{.05} = 1.761
_{ } = _{ }
2.45 2.15 ($.30 to $4.60)
We are 90% confident that the population mean price per share has decreased between $.30 and $4.60 over the four month period.
c. Sample mean price per share January 1: _{ }
Percentage decrease over the 4 months: _{ }
d. Mean price per share December 31, 2009 = $30.73(.92)(.92)(.92) = $23.93. This is a decline of
$30.73 – 23.93 = $6.80 per share for the year.
43. a. p_{1} = population proportion for men
p_{2} = population proportion for women
H_{0}: p_{1}  p_{2} = 0
H_{a}: p_{1}  p_{2} 0
b. _{ } = 248/800 = .31
_{ } = 156/600 = .26
c. _{ }
_{ }
pvalue = 2(1.0000  .9793) = .0414
pvalue _{ } .05, reject H_{0}. Conclude the population proportions are not equal. The proportion is higher for men.
d. _{ }
_{ }
.05 .0475
Margin of Error = .0475
95% Confidence Interval (.0025 to .0975)
44. a. _{ }= 76/400 = .19
_{ }= 90/900 = .10
_{ }
_{ }
pvalue 0
Reject H_{0}; there is a difference between claim rates.
b. _{ }
_{ }
.09 .0432 (.0468 to .1332)
Claim rates are higher for single males.
45. _{ }= 9/142 = .0634
_{ }= 5/268 = .0187
_{ }
_{ }
pvalue = 2(1.0000  .9911) = .0178
pvalue_{ } .02, reject H_{0}. There is a significant difference in drug resistance between the two states. New Jersey has the higher drug resistance rate.
46. a. March, 2007: _{ }= 70/200 = .35
March, 2008: _{ }= 70/150 = .47
b. _{ }
_{ }
Confidence interval: .12 1.96(.0529) or .12 .1037 (.0163 to .2237)
c. Since the confidence interval in part (b) does not include 0, I would conclude that occupancy rates are higher in the first week of March, 2008 than in the first week of March, 2007. On the basis of this I would expect occupancy rates to be higher for March, 2008 than for March, 2007.
47. _{ } Most recent week
_{ } One Week Ago
_{ }One Month Ago
a. Point estimate =_{ }
Margin of error: _{ }
95% confidence interval: .211 ± .085 (.296, .126)
b. H_{0}: p_{1} – p_{3} ≥ 0
H_{a}: p_{1} – p_{3} < 0
c. _{ }
_{ }
_{ }
pvalue = .0025
With pvalue ≤ .01, we reject H_{0} and conclude that bullish sentiment has declined over the past month.
10 
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