thesis - Faculty of Information and Communication Technologies ...

Chapter 5. Growth Dynamics
mentally, with new functionality constructed on top of existing code.
Though not surprising the real value of our analysis is that it provides
empirical support for the Yule process [210] also known as preferential
attachment growth model. In the simple form of this growth model, as
classes are added, the probability that a class will depend on another
existing class is proportional to the popularity of the existing class [55].
Simply put, popular classes will gain additional dependents causing
their popularity to increase. The preferential attachment model has
been used as one of the explanations for how certain web sites gain
popularity on the world-wide web [16, 209]. An interesting statistical
property of systems that exhibit growth via a preferential attachment
is that the data exhibits a highly skewed distribution and tends to fit a
power-law distribution [210].
Many researchers that study software have noted that the In-Degree
Count metric distributions follow a power-law [21,54,55,223,287,299],
and have hypothesised that this distribution arises due to preferential
attachment. That is, they have inferred the potential growth mechanism
from the observed outcome. However, although preferential attachment
can cause a power-law distribution, it is not the only model
that can give rise to this category of skewed distributions [48,145]. This
possibility implies that the hypothesis of preferential attachment model
generating skewed In-Degree Count distributions was not fully validated
empirically. Our observations show that, in general, there is an upward
trend in the Gini coefficient for IDC providing empirical support for the
preferential attachment growth model as the most likely explanation for
the observed highly skewed distributions for In-Degree Count metric.
In our data, we observed three systems – Struts, Tapestry, and Ant
where the Gini Coefficients for In-Degree Count decreased over time.
Why are these systems different? In order to answer this specific question,
we had to study the release notes as well as all available architecture
and design documentation for these systems.
The developers of Struts and Tapestry performed major restructuring
of the code base and in both cases, abandoned a substantial amount
of the code base during these restructuring phases (more detail is pre-
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