In 2004 and 2005, use of aggressive tax services provided by a company's auditor had become so broadly concerning that it was the focus of a PCAOB roundtable and discussed in a Congressional subcommittee investigation report (PCAOB, 2004 and US Senate, 2005). Although auditor provision of these and other nonaudit services to issuer audit clients was restricted in 2006, research on the effectiveness of these restrictions finds that they did not impact audit quality (Notbohm, Paterson and Valencia, 2015 and Lennox, 2016). We reexamine this issue with a focus on the audit quality effects for the engagements most impacted by the restrictions-Big 4 audit clients with pre-restriction tax service fees of at least $100,000 that fell by at least 75% following the restrictions. Using a difference-in-difference framework and two proxies for audit quality, we find evidence of the effectiveness of the PCAOB's 2006 restrictions among those clients. Additionally, we find these results are sensitive to the level of pre-restriction tax service fees, with the restatement (going concern) effect of the restrictions strengthening (weakening) in the pre-restriction tax service fee level. We also find that the effects of these restrictions are concentrated among clients of Big 4 auditors rather than clients of the 2nd tier or 3rd tier auditors. Results of additional analyses indicate that audit quality, as measured by the probability of restatement, was lower in the pre-restriction period for purchasers than for non-purchasers. Our results are robust to a barrage of sensitivity tests. Our findings contribute to the continued regulator discussion about the proper level and types of allowable tax nonaudit services.

在 2004 年和 2005 年，公司审计师提供的激进税收服务的使用变得如此广泛，以至于它成为 PCAOB 圆桌会议的焦点，并在国会小组委员会调查报告中进行了讨论（PCAOB，2004 年和美国参议院，2005 年）。尽管审计师向发行人审计客户提供这些和其他非审计服务在 2006 年受到限制，但对这些限制有效性的研究发现它们并未影响审计质量（Notbohm、Paterson 和 Valencia，2015 年和 Lennox，2016 年）。我们重新审视这个问题，重点关注受限制影响最大的业务的审计质量影响 - 限制前税务服务费至少为 100,000 美元的四大审计客户，在限制后下降了至少 75%。使用差异中的差异框架和两个审计质量代理，我们发现了 PCAOB 2006 年限制对这些客户的有效性的证据。此外，我们发现这些结果对限制前税收服务费水平敏感，限制的重述（持续经营）效应在限制前税收服务费水平中加强（减弱）。我们还发现，这些限制的影响集中在四大审计师的客户身上，而不是二级或三级审计师的客户身上。额外分析的结果表明，以重述概率衡量的审计质量，在限制前期间，买方的审计质量低于非买方的审计质量。我们的结果对一连串的敏感性测试是稳健的。