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Chapter 6. Change Dynamics
6.2.2 Rate of Modification
As noted in the previous section, a relatively small set of classes in
any given version is subject to change. However, how often are these
classes modified? What does the distribution of this modification look
like? In order to understand this better, we investigated the number
of times a class is modified. We ignore the total number of releases
for a project intentionally as it will reveal if there is a common pattern
between systems at an absolute level. For example, we can answer the
question - what is the probability that developers will modify a class more
than 3 times? In essence, we are making the assumption that after
a certain point (say the first few releases) the maturity of a software
system will have a minimal impact on the number of times a class is
modified.
For the analysis, we measured the number of times a class has been
modified since birth across the entire data set (approximately 55300
classes across all projects). In our data, on average 45% of the classes
were never modified once created. However, when the data was analyzed
by project, the range of classes that were never modified was
between 22% and 63%. For instance, in Saxon only 22% of the classes
were unchanged, while at the other end in Jung and ActiveMQ 63%
of the classes were unchanged after birth. Only considering classes
that have been modified at some point in their life cycle and counting
the number of times that they have been changed, we can observe that,
approximately 60% of the classes are modified less than 4 times (see
Figure 6.5). The range is however fairly broad. In Jung, for example,
90% of modified classes are changed less than 3 times.
Furthermore, though most systems have a similar modification profile
(see Figure 6.5), a few systems (Axis, iText, Saxon, Jung and ActiveBPEL)
show up as clear outliers. Axis, iText and Saxon had relatively
higher number of classes that underwent modification, we noticed that
Jung and ActiveBPEL were at the other extreme with fewer classes that
underwent changes. In 33 systems less than 10% of all modified classes
were changed more than 8 times. This suggests that the probability that
a class is modified multiple times is quite low and that this probability
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