Audits are often required by bonding companies, outside investors, and banks. Audits require the independent accountant to express an opinion that the financial statements have been subjected to testing and outside verification within rules established by the American Institute of Certified Public Accountants. The internal control of the company must be evaluated to determine the degree of testing required and to advise the client how to improve control weaknesses. The auditor's opinion is a higher level of assurance to those using the financial statements than reviews or compilations.