Women head up a low percentage of the audits of major public companies by Big Four firms, according to a report from the CFA Institute.

The study found that only 15 percent of lead engagement partners of the SP 500 were female, while only 11 percent of lead engagement partners of the SP 100 were female. The percentage of female lead engagement partners on SP 500 company audits was 20.8 percent at Deloitte, 16.3 percent for PricewaterhouseCoopers, 12.9 percent at Ernst Young and 10.6 percent for KPMG.

Thirty-one percent of the SP 500 were audited by EY, 30 percent by PwC, 20 percent by Deloitte, and 19 percent by KPMG, while 32 percent of the SP 100 were audited by PwC, 28 percent by EY, 24 percent by Deloitte, and 16 percent by KPMG.

“Although PwC has the greatest number of SP 500 clients, only 16.3 percent of them are serviced by female lead engagement partners, behind Deloitte’s 20.8 percent,” wrote Sandra Peters, head of financial reporting policy at the CFA Institute. “EY, with nearly the same number of SP 500 clients as PwC, is at 12.9 percent female lead engagement partners. We found no female partners among the 36 longest-tenured engagements (those over 75 years) in the SP 500. We found only six female partners in the 107 companies with auditor relationships exceeding 40 years.”

Women have claimed a number of top leadership roles with the Big Four recently, including Cathy Engelbert at Deloitte, Lynne Doughtie at KPMG and Kelly Grier at EY.

The study drew on information that the Securities and Exchange Commission and the Public Company Accounting Oversight Board now require from companies about their auditors, such as audit tenure. The CFA Institute used CalcBench software to analyze the audit tenure information. However, because disclosure of audit tenure wasn’t required for all companies with a year-end prior to Dec. 15, 2017, only 409 of the SP 500 companies had provided that information as of the writing of the report.