350 Statistical Methods
the correlation between the month
number and the average draft number.
How do the values of the correlation
in this analysis compare with those of
the correlation you performed earlier?
f. Create a scatter plot of average draft
number versus month number. Add a
trend line and include the regression
equation and R^2 value. How much
of the variability in the average draft
number per month is explained by the
month?
g. Save your changes to the workbook
and write a report summarizing your
conclusions. Which analysis (looking
at daily values or looking at monthly
averages) better describes any prob-
lem with the draft lottery?- The Emerald health care providers
claim that components of their health
plan cause it to rise signifi cantly more
slowly than overall health costs. You
decide to investigate to see whether
there is evidence for Emerald’s claim.
You have recorded Emerald costs over
the past seven years, along with the
consumer price index (CPI) for all urban
consumers and the medical component
of the CPI.
a. Open the Emerald workbook from
the Chapter08 folder and save it as
Emerald Regression Analysis.
b. Using the Analysis ToolPak’s Regres-
sion command, calculate the regres-
sion equation for each of the three
price indexes against the year vari-
able. What are the values for the three
slopes? Express the slope in terms
of the increase in the index per year.
How does Emerald’s change in cost
compare to the other two indexes?
c. Look at the 95% confi dence intervals
for the three slopes. Do the confi dence
intervals overlap? Does there appear
to be a signifi cant difference in the
rate of increase under the Emerald
plan as compared to the increases
under the other two indexes?
d. Summarize your conclusions. Do you
see evidence to substantiate Emerald’s
claim?
e. Save your changes and write a report
summarizing your observations.- The Teacher workbook contains data on
the relationship between teachers’ sala-
ries and the spending on public schools
per pupil in 1985. Perform the following
analysis on this data set:
a. Open the Teacher workbook from
the Chapter08 folder and save it as
Teacher Salary Analysis.
b. Create a scatter plot of spending per
pupil versus teacher salary. Add a
trend line containing the R^2 value and
regression to the plot.
c. Compute the regression statistics for
the data, and then create the diagnos-
tic plots discussed in this chapter. Is
there any evidence of a problem in
the diagnostic plots?
d. Copy the spending per pupil versus
teacher salary scatter plot to a new
chart sheet and then break down the
points in the plot on the basis of the
values of the area variable. For each
of the three series in the chart, add a
linear trend line and compute the
R^2 value and regression equation. How
do the least-squares lines compare
among the three regions? What do you
think accounts for any difference in
the trend lines?
e. Redo the regression statistics, per-
forming three regressions, one for
each of the three areas in the data set.
Compare the regression equations.
What are the 95% confi dence inter-
vals for the slope parameters in the
three areas?
f. Save your changes to the workbook
and write a report summarizing your
observations.