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.