Most of the auditor’s work, when it is forming an opinion is to obtain and evaluate audit evidence. Audit procedures for obtain audit evidence may include inspection, observation, confirmation, recalculation, reexecution, and analytical procedures, often combined with each other, in addition to inquiry. Although the inquiry may provide important audit evidence, and may even provide evidence of an incorrectness, it does not, on its own, provide sufficient audit evidence about the absence of material misstatement in assertions, nor about efficacy.
Operational controls, which is why it must be accompanied and / or supported by other audit procedures.
As explained in ISA 200, reasonable assurance is reached when the auditor has obtained sufficient and adequate audit evidence to reduce audit risk (ie, the risk of the auditor expressing an inadequate opinion when the financial statements contain material inaccuracies) in a acceptably low level.