Which type of evidence is most reliable for an IS auditor?

A confirmation letter received from a third party verifying an account balance is the most reliable type of evidence for an IS auditor due to its objectivity and independence. Third-party confirmations are considered strong evidence because they come from an external source that is not influenced by the organization being audited. This reduces the risk of bias and increases the credibility of the information provided.

In auditing practices, reliability is paramount, and independent evidence is typically valued more highly than evidence obtained from within the organization. In this case, a confirmation letter serves this purpose effectively as it directly substantiates account balances and can be traced back to an external party validating the information against official records.

Other types of evidence, such as assurance from line management or trend data from Internet sources, may lack the same level of independence and can be influenced by internal perceptions or external accuracy. Ratio analysis developed by the IS auditor, while useful, is also based on internally sourced reports and may not provide the same level of assurance as third-party verification. Thus, while these other options may have some value, they do not match the level of reliability provided by a confirmation letter from a third party.