“…8 In the above simple example, firm B's clients in the mining industry belong to the treatment group, while all other clients belong to the control group. 9 We employ a difference-in-difference approach and examine whether the audit quality for the treatment group improves after the merger (three-year post-merger versus three-year pre- 7 The existence of individual heterogeneity within the same audit firm is well documented in prior research (e.g., Libby 1981;Bonner and Lewis 1990;Libby and Tan 1994;Gibbins and Swieringa 1995;Bonner 2008;Nelson 2009;Gul, Wu and Yang 2013;Aobdia, Lin and Petacchi 2015;Ke, Lennox and Xin 2015;Knechel, Vanstraelen, and Zerni 2015;Li, Qi, Tian and Zhang 2017;He, Kothari, Xiao and Zuo 2018;Lennox and Wu 2018). 8 Archival auditing research commonly uses industry market share to measure expertise and generally finds consistent evidence (DeFond and Zhang 2014).…”