348 CHAPTER 10 STATISTICAL INFERENCE FOR TWO SAMPLESand s 2 3 F. Do the sample data support the claim
that both alloys have the same melting point? Use 0.05 and
assume that both populations are normally distributed and have
the same standard deviation. Find the P-value for the test.
10-26. Referring to the melting point experiment in
Exercise 10-25, suppose that the true mean difference in
melting points is 3F. How large a sample would be required
to detect this difference using an 0.05 level test with
probability at least 0.9? Use 1 2 4 as an initial esti-
mate of the common standard deviation.
10-27. Two companies manufacture a rubber material in-
tended for use in an automotive application. The part will be
subjected to abrasive wear in the field application, so we
decide to compare the material produced by each company in
a test. Twenty-five samples of material from each company
are tested in an abrasion test, and the amount of wear after
1000 cycles is observed. For company 1, the sample mean and
standard deviation of wear are milligrams /1000
cycles and s 1 2 milligrams/1000 cycles, while for company
2 we obtain milligrams/1000 cycles and s 2 8 mil-
ligrams/1000 cycles.
(a) Do the data support the claim that the two companies pro-
duce material with different mean wear? Use 0.05,
and assume each population is normally distributed but
that their variances are not equal.
(b) What is theP-value for this test?
(c) Do the data support a claim that the material from com-
pany 1 has higher mean wear than the material from com-
pany 2? Use the same assumptions as in part (a).
10-28. The thickness of a plastic film (in mils) on a sub-
strate material is thought to be influenced by the temperature
at which the coating is applied. A completely randomized ex-
periment is carried out. Eleven substrates are coated at 125F,
resulting in a sample mean coating thickness of
and a sample standard deviation of s 1 10.2. Another 13 sub-
strates are coated at 150F, for which and s 2 20.1
are observed. It was originally suspected that raising the
process temperature would reduce mean coating thickness. Do
the data support this claim? Use 0.01 and assume that the
two population standard deviations are not equal. Calculate an
approximate P-value for this test.
10-29. Reconsider the coating thickness experiment in
Exercise 10-28. How could you have answered the question
posed regarding the effect of temperature on coating thickness
by using a confidence interval? Explain your answer.
10-30.Reconsider the abrasive wear test in Exercise 10-27.
Construct a confidence interval that will address the questions
in parts (a) and (c) in that exercise.
10-31. The overall distance traveled by a golf ball is tested
by hitting the ball with Iron Byron, a mechanical golfer with a
swing that is said to emulate the legendary champion, Byron
Nelson. Ten randomly selected balls of two different brands
are tested and the overall distance measured. The data follow:x 2 99.7x 1 103.5x 2 15x 1 20(a) Do the data support the claim that the mean etch rate is the x 2 426 F,
same for both solutions? In reaching your conclusions, use
0.05 and assume that both population variances are
equal.
(b) Calculate a P-value for the test in part (a).
(c) Find a 95% confidence interval on the difference in mean
etch rates.
(d) Construct normal probability plots for the two samples.
Do these plots provide support for the assumptions of nor-
mality and equal variances? Write a practical interpreta-
tion for these plots.
10-22. Two suppliers manufacture a plastic gear used in a
laser printer. The impact strength of these gears measured in
foot-pounds is an important characteristic. A random sample
of 10 gears from supplier 1 results in and s 1 12,
while another random sample of 16 gears from the second
supplier results in and s 2 22.
(a) Is there evidence to support the claim that supplier 2 pro-
vides gears with higher mean impact strength? Use
0.05, and assume that both populations are normally dis-
tributed but the variances are not equal.
(b) What is the P-value for this test?
(c) Do the data support the claim that the mean impact
strength of gears from supplier 2 is at least 25 foot-pounds
higher than that of supplier 1? Make the same assump-
tions as in part (a).
10-23. Reconsider the situation in Exercise 10-22, part (a).
Construct a confidence interval estimate for the difference in
mean impact strength, and explain how this interval could
be used to answer the question posed regarding supplier-
to-supplier differences.
10-24. A photoconductor film is manufactured at a nominal
thickness of 25 mils. The product engineer wishes to increase
the mean speed of the film, and believes that this can be
achieved by reducing the thickness of the film to 20 mils.
Eight samples of each film thickness are manufactured in a pi-
lot production process, and the film speed (in microjoules per
square inch) is measured. For the 25-mil film the sample data
result is and s 1 0.11, while for the 20-mil film,
the data yield and s 2 0.09. Note that an increase
in film speed would lower the value of the observation in mi-
crojoules per square inch.
(a) Do the data support the claim that reducing the film thick-
ness increases the mean speed of the film? Use 0.10
and assume that the two population variances are equal
and the underlying population of film speed is normally
distributed.
(b) What is the P-value for this test?
(c) Find a 95% confidence interval on the difference in the
two means.
10-25. The melting points of two alloys used in formulating
solder were investigated by melting 21 samples of each material.
The sample mean and standard deviation for alloy 1 was
x 1 420 F and s 1 4 F, while for alloy 2 they werex 2 1.06x 1 1.15x 2 321x 1 290c 10 .qxd 5/16/02 1:31 PM Page 348 RK UL 6 RK UL 6:Desktop Folder:TEMP WORK:MONTGOMERY:REVISES UPLO D CH114 FIN L:Quark Files: