The reliability of audit evidence is influenced by its source internal or external and by its visual, documentary or oral while the reliability of audit evidence is dependent on the circumstances under which it is obtained the following generalizations as laid down in NSA 500, Audit Evidence, may be useful in assessing the reliability of audit evidence:
External evidence (e.g. confirmation received from a third party) is more reliable than internal evidence.
Internal evidence is more reliable when related internal control is satisfactory.
Evidence obtained by the auditor himself is more reliable than that obtained from the entity.
Evidence in the form of documents and written representations is usually more reliable than oral representations.