Stat 301
HW 3
Due: 18 Sept / 21 Sept 2014
Note: In problems 1 and 2, use equal-variance methods. In problem 3, decide whether it is more
appropriate to use equal or unequal variance methods.
1. Text problem 1.68 (p. 61). Ignore the book’s questions. Instead answer and show your work:
(a) The text has summary statistics (e.g., average, standard deviation) for the two groups
(oiled and unoiled) on page 62. Use the information to calculate the pooled standard
deviation (i.e., the square root of the pooled variance, s2p ).
(b) Calculate the degrees of freedom for the pooled standard deviation.
2. Text problem 1.66 (p. 61). Ignore the book’s questions. The data are in bulimia.txt. Instead,
answer:
(a) Is it reasonable to assume equal variances? Explain why or why not.
(b) What is the pooled standard deviation,
q
s2p ?
(c) What is the 95% confidence interval for the difference in means?
(d) Based on the confidence interval, what can you say about the p-value for a two-sided
test that the two groups have the same mean.
(e) Have JMP test whether the two groups have the same mean. What is the p-value?
3. Text problem 1.109 (p. 78). Read the book’s description, but ignore its questions. The data
in the file Milk1987.csv are from a sample of school districts in the tri-county area and a
sample of school districs in the surrounding area. As the text for the problem indicates, these
data were collected to investigate potential price-fixing in the tri-county market, which will
be indicated by a higher mean bid price in the tri-county area.
Examine the data, analyze them in an appropriate manner, then write a short paragraph that
summarizes what you have learned about milk bid prices in these two regions of Kentucky.
You are encouraged to include a table or figure if that helps you summarize what you have
learned.
Remember the goal of your analysis is to make conclusions about the difference in milk prices
between the two regions.
1