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Chapter 6. Change Dynamics
The set of changed classes C v and the set of unchanged classes U v are
determined by comparing the classes in two releases using the change
detection technique presented in Section 6.1.2.
The set of deleted classes Dv f in the next release is determined by comparing
any given version with the future version (if available) and checking
if the fully qualified class name exists in both versions. The set of
newly added classes A v is computed by comparing any given version
with the previous version (if available) and checking if the fully qualified
class name exists in that version.
6.1.4 Measuring Change
Once the set of changed classes is established, three different dimensions
of change can be computed: amplitude (size of change), periodicity
(frequency of change), and dispersion (consistency of change). These
dimensions were proposed by Barry et al. [17] based on a study into
volatility in software systems where a phase sequence analysis of maintenance
log data (revision history) was performed. We based our study
on similar dimensions, however we use release history (see Section 3.1)
as our input and hence can not directly apply the method proposed by
Barry et al. [17].
In this study, we use the following definitions for the dimensions of
change — the size of change between two classes is defined as the number
of metrics that differ (i.e., if five different metrics differ in value between
two classes, then the size of change is 5). The modification count
is the number of times a class has been modified since its creation —
this measure is used as the basis for determining the periodicity. Finally,
in order to assess dispersion of change, we measure the proportion
of classes in the final version that remain unchanged since creation
as well as the proportion that are modified after being created.
We measure the size, consistency and frequency of change for each class
in our release history (that is, every class in every version). The rationale
for our choices is presented in the next section within the context
of the observations.
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