Abstract  In this paper, we analyze the data extracted from several open source software repositories. We observe that the change data follows a Zipf distribution. Based on the extracted data, we then develop three probabilistic models to predict which files will have changes or bugs. The first model is Maximum Likelihood Estimation (MLE), which simply counts the number of events, i.e., changes or bugs, that happen to each file and normalizes the counts to compute a probability distribution. The second model is Reflexive Exponential Decay (RED) in which we postulate that the predictive rate of modification in a file is incremented by any modification to that file and decays exponentially. The third model is called REDCoChange. With each modification to a given file, the REDCoChange model not only increments its predictive rate, but also increments the rate for other files that are related to the given file through previous cochanges. We then present an informationtheoretic approach to evaluate the performance of different prediction models. In this approach, the closeness of model distribution to the actual unknown probability distribution of the system is measured using cross entropy. We evaluate our prediction models empirically using the proposed informationtheoretic approach for six large open source systems. Based on this evaluation, we observe that of our three prediction models, the REDCoChange model predicts the distribution that is closest to the actual distribution for all the studied systems.
