Putting It Together: Time to Complete a Degree A researcher wanted to
determine if the mean time to complete a bachelor's degree was different
depending on the selectivity of the first institution of higher education that
was attended. The following data represent a random sample of 12 th-graders
who earned their degree within eight years. Probability plots indicate that
the data for each treatment level are normally distributed.
$$
\begin{array}{ccccc}
\text { Highly } & & & \text { Not } \\
\text { Selective } & \text { Selective } & \text { Nonselective } & \text {
Open-door } & \text { Rated } \\
\hline 2.5 & 4.6 & 4.7 & 5.9 & 5.1 \\
\hline 5.1 & 4.3 & 2.3 & 4.2 & 4.3 \\
\hline 4.3 & 4.4 & 4.3 & 6.4 & 4.4 \\
\hline 4.8 & 4.0 & 4.2 & 5.6 & 3.8 \\
\hline 5.5 & 4.1 & 5.1 & 6.0 & 4.7 \\
\hline 2.6 & 3.1 & 4.7 & 5.0 & 4.5 \\
\hline 4.1 & 3.8 & 4.2 & 7.3 & 5.5 \\
\hline 3.4 & 5.2 & 5.7 & 5.6 & 5.5 \\
\hline 4.3 & 4.5 & 2.3 & 6.9 & 4.6 \\
\hline 3.9 & 4.0 & 4.5 & 5.1 & 4.1 \\
\hline
\end{array}
$$
(a) What type of observational study was conducted? What is the response
variable?
(b) Find the sample mean for each treatment level.
(c) Find the sample standard deviation for each treatment level. Using the
general rule presented in this chapter, does it appear that the population
variances are the same?
(d) Use the time to degree completion for students first attending highly
selective institutions to construct a \(95 \%\) confidence interval estimate for
the population mean.
(e) How many pairwise comparisons are possible among the treatment levels?
(f) Consider the null hypothesis \(H_{0}:
\mu_{1}=\mu_{2}=\mu_{3}=\mu_{4}=\mu_{5}\). If we test this hypothesis using \(t\)
-tests for each pair of treatments, use your answer from part (e) to compute
the probability of making a Type I error, assuming that each test uses an
\(\alpha=0.05\) level of significance.
(g) Use the one-way ANOVA procedure to determine if there is a difference in
the mean time to degree completion for the different types of initial
institutions. If the null hypothesis is rejected, use Tukey's test to
determine which pairwise differences are significant using a familywise error
rate of \(\alpha=0.05\)