In issue tracking systems, each bug is assigned a priority level (e.g., Blocker, Critical, Major, Minor, or Trivial in JIRA from highest to lowest), which indicates the urgency level of the bug. In this sense, understanding bug priority changes helps to arrange the work schedule of participants reasonably, and facilitates a better analysis and resolution of bugs. According to the data extracted from JIRA deployed by Apache, a proportion of bugs in each project underwent priority changes after such bugs were reported, which brings uncertainty to the bug fixing process. However, there is a lack of indepth investigation on the phenomenon of bug priority changes, which may negatively impact the bug fixing process. Thus, we conducted a quantitative empirical study on bugs with priority changes through analyzing 32 non-trivial Apache open source software projects. The results show that: (1) 8.3% of the bugs in the selected projects underwent priority changes; (2) the median priority change time interval is merely a few days for most (28 out of 32) projects, and half (50. 7%) of bug priority changes occurred before bugs were handled; (3) for all selected projects, 87.9% of the bugs with priority changes underwent only one priority change, most priority changes tend to shift the priority to its adjacent priority, and a higher priority has a greater probability to undergo priority change; (4) bugs that require bug-fixing changes of higher complexity or that have more comments are likely to undergo priority changes; and (5) priorities of bugs reported or allocated by a few specific participants are more likely to be modified, and maximally only one participant in each project tends to modify priorities.

翻译：在问题跟踪系统中，每个缺陷都被分配了一个优先级（例如JIRA中从高到低的Blocker、Critical、Major、Minor或Trivial），用以指示缺陷的紧急程度。因此，理解缺陷优先级变更有助于合理规划参与者的工作安排，并促进缺陷的更好分析与解决。根据从Apache部署的JIRA中提取的数据，每个项目中有一部分缺陷在报告后发生了优先级变更，这给缺陷修复过程带来了不确定性。然而，目前缺乏对缺陷优先级变更现象的深入调查，这可能对缺陷修复过程产生负面影响。为此，我们通过分析32个非平凡的Apache开源软件项目，对发生优先级变更的缺陷进行了定量实证研究。结果表明：(1) 所选项目中8.3%的缺陷发生了优先级变更；(2) 大多数项目（32个中的28个）的优先级变更时间间隔中位数仅为几天，且半数（50.7%）的缺陷优先级变更发生在缺陷被处理之前；(3) 在所有所选项目中，87.9%的优先级变更缺陷仅经历一次变更，大多数优先级变更倾向于将优先级调整至相邻级别，较高优先级更易发生变更；(4) 需要更高复杂度的缺陷修复变更或拥有更多评论的缺陷更可能经历优先级变更；(5) 由少数特定参与者报告或分配的缺陷优先级更易被修改，且每个项目中最多仅有一名参与者倾向于修改优先级。