Page 191 - Statistics II for Dummies

Chapter 10: Sorting Out the Means with Multiple Comparisons 175

Table 10-1 Cellphone Minutes Used in One Month
19 and Under 20–39 (Group 2) 40–59 (Group 3) 60 and Over
(Group 1) (Group 4)
800 250 700 200
850 350 700 120
800 375 750 150
650 320 650 90
750 430 550 20
680 380 580 150
800 325 700 200
750 410 700 130
690 450 590 160
710 390 650 30

The means and standard deviations of the sample data are shown in Figure
10-1, as well as confidence intervals for each of the population means sepa-
rately (see Chapter 3 for info on confidence intervals). Looking at Figure
10-1, it appears that all four means are different, with the 19-and-under group
heading the pack, 40- to 59-year-olds not far behind, with 20- to 39-year-olds
and those over 60 bringing up the rear (in that order).
Knowing that you can’t live by sample results alone, you decide that ANOVA
is needed to see whether any differences that appear in the samples can be
extended to the population (see Chapter 9). By using the ANOVA procedure,
you test whether the average cell minutes used is the same across all groups.
The results of the ANOVA, using the data from Table 10-1, are shown in
Figure 10-2.

Individual 95% CIs For Mean Based on
Figure 10-1: Pooled StDev
Basic sta-
Level N Mean StDev ––––––+–––––––––+–––––––––+–––––––––+–––
tistics and
Group 1 10 748.00 64.60 (–*–)
confidence Group 2 10 368.00 59.08 (–*–)
intervals Group 3 10 657.00 64.99 (–*–)
Group 4 10 125.00 62.41 (–*–)
for the cell- ––––––+–––––––––+–––––––––+–––––––––+–––
phone data. 200 400 600 800

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