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52 CHAPTER 2 DESCRIPTIVE STATISTICS
Dialog box:
Stat ➤ EDA ➤ Boxplot ➤ Simple
Click OK.
Type Force Graph Variables.
Click OK.
Session command:
MTB > Boxplot ‘Force’;
SUBC> IQRbox;
SUBC> Outlier.
FIGURE 2.5.7 MINITAB procedure to produce Figure 2.5.6.
The SAS ® statement PROC UNIVARIATE may be used to obtain a box-andwhisker
plot. The statement also produces other descriptive measures and displays,
including stem-and-leaf plots, means, variances, and quartiles.
Exploratory Data Analysis Box-and-whisker plots and stem-and-leaf displays
are examples of what are known as exploratory data analysis techniques. These
techniques, made popular as a result of the work of Tukey (3), allow the investigator to
examine data in ways that reveal trends and relationships, identify unique features of
data sets, and facilitate their description and summarization.
EXERCISES
For each of the data sets in the following exercises compute (a) the mean, (b) the median, (c) the
mode, (d) the range, (e) the variance, (f) the standard deviation, (g) the coefficient of variation,
and (h) the interquartile range. Treat each data set as a sample. For those exercises for which you
think it would be appropriate, construct a box-and-whisker plot and discuss the usefulness in understanding
the nature of the data that this device provides. For each exercise select the measure of
central tendency that you think would be most appropriate for describing the data. Give reasons
to justify your choice.
2.5.1 Porcellini et al. (A-8) studied 13 HIV-positive patients who were treated with highly active antiretroviral
therapy (HAART) for at least 6 months. The CD4 T cell counts 1* 10 6 >L2 at baseline for the
13 subjects are listed below.
230 205 313 207 227 245 173
58 103 181 105 301 169
Source: Simona Porcellini, Guiliana Vallanti, Silvia
Nozza, Guido Poli, Adriano Lazzarin, Guiseppe
Tambussi, Antonio Grassia, “Improved Thymopoietic
Potential in Aviremic HIV Infected Individuals
with HAART by Intermittent IL-2 Administration,”
AIDS, 17 (2003), 1621–1630.
2.5.2 Shair and Jasper (A-9) investigated whether decreasing the venous return in young rats would affect
ultrasonic vocalizations (USVs). Their research showed no significant change in the number of
ultrasonic vocalizations when blood was removed from either the superior vena cava or the carotid