User Guide

495
Nonparametric
Te s t s
Two-Indepen
dent-Samples Tests
The Two-Ind
ependent-Samples Tests procedure compares two groups of cases on
one variable.
Example. New dental braces have been developed that are intended to be more
comfortabl
e, to look better, and to provide more rapid progress in realigning teeth. To
find out if the new braces have to be worn as long as the old braces, 10 children are
randomly chosen to wear the old braces, and another 10 are chosen to wear the new
braces. Fr
om the Mann-Whitney U test, you might find that, on average, those with
the new braces did not have to wear the braces as long as those with the old braces.
Statistics. Mean, standard deviation, minimum, maximum, number of nonmissing
cases, and
quartiles. Tests: Mann-Whitney U, Moses extreme reactions,
Kolmogorov-Smirnov Z, Wald-Wolfowitz runs.
Data. Use numeric variables that can be ordered.
Assumpti
ons.
Use independent, random samples. The Mann-Whitney U test requires
that the two samples tested be similar in shape.
Figure 34
-13
Two-Inde
pendent-Samples output
10 14.10 141.00
10 6.90 69.00
20
Old
Braces
New
Braces
Total
Type of
Braces
Time
Worn in
Days
N
Mean
Rank
Sum of
Ranks
Ranks