The Public Company Accounting Oversight Board inspected 34 audit files at Grant Thornton during its 2017 inspections of 2016 audit work. It found problems in six audits serious enough to describe in its latest report on the firm, for a deficiency rate of 18 percent.

That’s down from 24 percent in 2016 and considerably lower than the firm’s toughest year, in 2012, when inspectors called out 65 percent of engagements. It also makes it the first major firm to deliver a rate in the teens since 2009.

Since the PCAOB toughened up its inspection process in 2010 and began delivering increasingly harsher results, none of the major firms have seen deficiency rates below the 20 percent earned by PwC in 2016 and then Deloitte in 2017. While rates have generally fallen the past few years at most firms, a few firms have continued to struggle.

KPMG, for example, delivered a 50 percent deficiency rate in its latest report as authorities pursued charges of fraud and conspiracy around an effort to subvert the inspections process. BDO USA has not earned a deficiency rate below 50 percent in five years.

“We are pleased with this year’s strong PCAOB audit inspection report, which shows our dedication to continuous improvement and reflects our firm’s commitment to delivering high-quality audits for our clients,” Grant Thornton said in a statement. “Quality is the foundation of all we do and will continue to be a principal focus at Grant Thornton.”

Of the six audits called out for deficiencies, five contained a total of 10 separate errors in the audit of internal control over financial reporting, according to the PCAOB’s report. Inspectors said most of the internal control audit problems focused on the testing of controls, including the design and operating effectiveness of controls, which has been a common theme in reports the past several years.

In terms of accounts most often presenting problems, auditors appeared to struggle more with revenue recognition, including accounts receivable, than other areas. Business combinations and taxes also proved problematic.