Based on its latest round of audit inspections in 2017, the Public Company Accounting Oversight Board says it found some improvement at some firms, but still plenty of work to do to meet standards.

The board published a 19-page brief summarizing its inspection approach in 2017, which examined the work of auditors on 2016 financial statements, along with some broad findings. The board says it headed into 2017 inspections with plans to dig into some of the historically problematic areas — especially audits of internal control over financial reporting, auditors’ assessments and responses to risks of material misstatements, and auditing of accounting estimates, including fair value measurements.

“While inspections staff observed improvements in the audit work performed at some firms in these areas in recent years, it continued to find high numbers of deficiencies at many firms,” the PCAOB says. “The persistent deficiencies related to these areas indicate that auditors should continue to consider these matters when planning and performing their audits.”

The six largest audit firms have received inspection results from the PCAOB the past few years that have characterized anywhere from 20 percent to 50 percent of the audits inspected as deficient. These are audit mistakes serious enough in the eyes of inspectors that auditors did not have enough evidence to issue the opinions they issued. The PCAOB’s latest round of published reports shows deficiency rates averaging more than 30 percent across those six firms.

The PCAOB’s latest report says inspectors continue to evaluate the process auditors follow for identifying and responding to risks of material misstatement at the financial statement assertion level. “Proper identification of the risks of material misstatement, including the types of potential misstatements that can occur and the likely sources of those potential misstatements, is necessary when selecting appropriate controls to test, evaluating whether those controls adequately address the risks, and designing and executing substantive procedures,” the board wrote.

The PCAOB’s report says inspectors have noted some decrease in deficiencies that arise when firms use the work of other auditors. Some firms have improved their methodologies or tools around multi-location audits and have improved their supervision and review of work they rely on from other firms. “While it is still to early to judge how effective these changes are, these efforts may have contributed to the recent decrease in audit deficiencies,” the board says.

The board says some firms are doing a better job than others at identifying root causes of audit failures, which might help explain differences in inspection outcomes across the firms. “Firms that have responded to recurring audit deficiencies with meaningful, carefully considered actions to address underlying issues and causes are beginning to see improved results,” the board says.