Discussion of Evaluation Methodologies

In previous research, there are plenty of arguments about textual backdoor evaluation, including diverse metrics and experiment settings. These valuable discussions motivate us to construct a rigorous benchmark and we highly appreciate their efforts. In this section, we briefly summarize existing opinions and provide a more detailed discussion on this topic. Table 9 summarizes the attackers OpenBackdoorimplements. Effectiveness Besides the mainstream ASR (also called LFR [20]) and CACC metrics, there are also other effectiveness metrics. Shen et al. [46] proposed to count the number of inserted triggers that can successfully flip the label. However, although inserting more triggers could benefit attack strength, the triggers also corrupt the sentences gradually, so it is also possible that the poisoned samples become "adversarial", and we can hardly distinguish. Shen et al. [45] also mentioned this issue, and they advised calculating the ASR difference between a poisoned model and a clean model as an effectiveness metric.