
252 Empirical Research in Software Engineering
use the Wilcoxon–Mann–Whitney test for comparing the differences among
coupling values of an academic and open source software.
Step 3: Apply test and calculate p-value.
In this example, n
1
=4,n
2
=5,and N=9. Table6.33shows the arrangement of
all the observations in ascending order, and the ranks allocated to them.
Sum of ranks assigned to observations in Academic software (T
1
)=4+6+8+9=27.
Sum of ranks assigned to observations in open source software
(T
2
)=1+2+3+5+7=18.
The U-statistic is given below:
U=n.n+
nn+
T
12
11
1
1
2
+
+1
−
=
()
45
44
27 3.
U=n.n+
nn+
T
12
22
2
1
2
=+
+1
−
()
−=