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Chapter 2. Software Evolution
added, removed and modified [85, 217]. Periodicity measures the regularity
at with a system or an abstraction is modified. For instance,
this measure is required to determine how often a file is modified over
the evolution history. The expectation is that if a large part of the code
base is modified frequently, then the software is highly volatile and may
require corrective action. Finally, the measure of dispersion aims to
identify if there is a consistent pattern to the change. This measure
is motivated by the assumption that consistency allows managers to
anticipate how much of the code base may change in the next version
and hence can allocate resources appropriately. The dispersion measure
can be applied to determine the consistency of the size of change,
as well as the consistency in the frequency of change. In this thesis,
we study change against these three dimensions. A discussion our approach
to compute change against these three dimensions is presented
in Chapter 6.
Studies of change that investigate these dimensions are able to provide
us with a baseline on what to expect in evolving software. Specifically,
we can identify periods of normal and abnormal change. Though an
understanding of these dimensions of change is useful [17], we found
comparatively few studies in the literature that have focused on a quantitative
analysis of these dimensions of change. Furthermore, most
studies investigated only a few software systems (typically under 10),
impacting on the strength of their findings.
An early study that presents observations from an investigation of the
frequency of change was undertaken by Kemerer et al. [147] who studied
the profile of software maintenance in five business systems at the
granularity of modules. They concluded that very few modules change
frequently, and later extended this study by identifying that the modules
that did change can be considered to be strategic [148] (in terms of
business functionality offered). Both of these studies inferred change
by analysing defect logs generated during the development of a commercial
non-object oriented software system. This study was not able
to establish the typical size of change, or the consistency of the change.
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