The primary role of an auditor, regardless of whether they are internal or external, is to review a company's financial statements and form an opinion regarding the company's financial position. An auditor's ultimate goal is to ensure that the company for which they are auditing can grasp the effectiveness of their processes and operations in order to set new goals and objectives for performance. Accounting statements reveal the success or failure of operations in a company so closely analyzing these can assist a company greatly in creating future goals. The roles of an internal and external auditor vary slightly.
An internal auditor's goal is to assist the company in analyzing and setting effective goals and objectives for performance. They also help the company make performance metrics for which to measure performance against. As the internal auditor is an employee of the company, they will have the company's best interest in mind when auditing. This does not mean that they can alter financial statements so that the company position looks improved.
An external auditor is not an employee of the company it is auditing, rather, they are usually hired by an external group that wishes to have an unbiased and objective opinion of the financial position of the company in question. An external auditor will not help the company it is auditing set goals for future performance. They must determine whether the financial statements are a fair representation of the actual financial position of the company.
Both types of auditors must do an extensive amount of research about the control systems and operations to gather sufficient evidence to form an opinion. It is important that an auditor do not overstep their boundaries regarding the actual operations of the company because this could render their opinion biased, therefore, becoming unreliable.