CSR reports gain more credibility if they are verified by external parties, much like financial statements become more credible when they are audited by independent CPAs. Whereas the SEC requires that publicly traded companies have their financial statements no such requirement for CSR reports. Nonetheless, 45% of GRI reports issued in 2011 were externally "assured “ Assurance is a term used to denote an independent party's external validation of management's assertions. The Big Four accounting firms are the leading assurance service providers for CSR reports. The Big Four firms hire many experts in other fields, such as engineers, to aid accountants in their assurance service practices.