Did the PCAOB's Restrictions on Auditors' Tax Services Improve Audit Quality?

In 2005-2006, the PCAOB imposed restrictions on auditors' tax services in order to strengthen auditor independence and improve audit quality. The restrictions resulted in a significant drop in auditor-provided tax services (APTS). To test the impact on audit quality, I partition the sample into a treatment group (companies whose APTS purchases dropped significantly when the restrictions were introduced) and a control group (companies whose APTS purchases were relatively unaffected) and I measure audit quality using the incidence of accounting misstatements, tax-related misstatements, and auditors' going-concern opinions. Using a difference-in-differences design, I find no change in audit quality for the treatment group relative to the control group after the restrictions are imposed.