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Chapter 6. Change Dynamics
ity with evolution? In order to answer these questions, we analysed the
proportion of classes that are popular p v (Equation 6.2.11), against the
proportion of all the popular classes that are new a p v (Equation 6.2.14).
a p v = |Ap v|
|A v |
(6.2.14)
In Equation 6.2.14, A p v is the set of new classes that are popular, and
A v is the set of new classes in version v.
As in the previous section, where we analyzed modified classes, to
ensure sufficient statistical power (in terms of our ability to compare
samples), we eliminate versions where less than 30 classes have been
added [271].
We observed the following property (Equation 6.2.15) holds, for 88% of
the releases in our data set:
p v > a p v (6.2.15)
And, the following property (Equation 6.2.16) holds, for 85% of the releases
in our data set:
c p v > p v > a p v (6.2.16)
The property captured by Equations 6.2.15 and 6.2.16 shows that, in
general, the new classes do not start out popular, and that classes that
tend to be modified are those that are popular. This property is illustrated
for the Spring Framework in Figure 6.7. Although, developers
create some new classes with a higher In-Degree, in general the proportion
of these classes is substantially lower than the proportion of
popular classes within the software system.
We observed that the In-Degree profile of the new classes is very different
from the profile of existing code. New classes tend to start with a
substantially lower In-Degree Count, and as they are modified over time
move towards the overall trend. If we compare the proportion of popular
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