Interest of Defect Technical Debt: An Exploratory Study on Apache Projects

Defect technical debt (defect debt) refers to known deferred bugs that have not been fixed. The interest of a defect debt item (i.e., bug) is the extra effort needed to fix the bug due to the delay of fixing. It is important to measure defect debt interest in a software system in order to be aware of how much the interest of certain bugs is and which bugs should be fixed first. Furthermore, it is valuable to understand the features of the bugs of high interest or no interest, so as to facilitate the identification of those kinds of bugs. In this work, we proposed three pairs of measures for quantifying defect debt interest at three granularities (i.e., lines of code, source file, and package) of software changes, and conducted an exploratory case study on 13,438 bugs collected from 59 non-trivial Apache open source software projects written mainly in Java. We have the following findings. (1) Each bug, with an average of 224 days delay of fixing, has interest of 660 to 845 lines of code, 0.57 to 1.05 source files, and 0.29 to 0.35 package on average in need for understanding or modifying when fixing the bug. (2) The average interest of a bug shows overall increasing trends over delayed time of bug fixing and bug priority levels from the lowest to highest. (3) Around 30% of the bugs under study did not incur interest at the granularity of lines of code, which means that the involved source files in the bug-fixing commits of such bugs were not modified at all during the delayed time of bug fixing. (4) The average code change size of bug-fixing commits of a bug without interest is much smaller than that of a bug with (high) interest; for a bug without interest at the granularity of lines of code, the source files involved in the bug-fixing commits of the bug were alternately modified by different committers much less frequently than those of the bug with (high) interest.