The Pollution from Obsolete Issue Report: An Empirical Study Project summary In software development, programmers use issue trackers to manage their maintenance issues and record valuable maintenance details in issue reports. Based on these issue reports, programmers have enhanced code comprehension and researchers have mined knowledge from issue reports to assist various programming tasks. Although issue reports are useful, some of them can be obsolete, in that their corresponding commits are overwritten or rolled back, with the evolution of software. The obsolete issue reports can invalidate their references and descriptions, and can have far-reaching impacts on the approaches built on them. To deepen the understanding of obsolete issue reports, we conducted the first empirical study to analyze obsolete issue reports. We consider that an issue report is obsolete if its revisions are partially or totally removed in later commits. To measure how an issue report becomes obsolete, we define an obsolete ratio of an issue report as its deleted lines over all its modified lines. In this paper, we build a tool, ICLinker, to inspect the obsolete issue reports and calculate the obsolete ratios. With ICLinker, we analyze 116,106 commits and 72,136 issue reports that are collected from 9 Apache projects. Taking them as our inputs, we explore four research questions, which concern the distributions, the references, the explanations and the impact of obsolete issue reports. Our findings on these research questions enrich the knowledge of obsolete issue reports, and some are even counterintuitive. For example, we find that obsolete issue reports are mixed with other issue reports. As another example, we find that only a small portion of issue reports are mentioned in code comments, but about half of them are obsolete. We confirm that 48.18% of issue reported recommended by a recent approach are obsolete. As many approaches take issue reports as inputs, obsolete issue reports can affect more research topics than recommending issue reports.