46. ECL is a complex accounting estimate that often involves interrelated processes across multiple areas of a bank and significant data and IT requirements, which means effective internal controls are needed to ensure the completeness, accuracy and reasonableness of the related information, from data collection and throughout complex processing. In response, a bank's external auditor performs appropriate tests on the controls over the ECL estimate to assess whether, and to what extent, the external auditor can rely on them [Basel Committee on Banking Supervision, The External audits of banks, March 2014, paragraph 154.].

47. Given the complexity, subjectivity, and high degree of estimation uncertainty around the estimate of ECL, and the fact that the ECL estimate can often involve models and multiple data points, substantive procedures alone will usually not provide sufficient appropriate audit evidence that the ECL estimate is not materially misstated. The external auditor should obtain an understanding of and evaluate the design and implementation of the bank's system of internal controls. For controls the external auditor plans rely upon, the external auditor should test the operating effectiveness and determine the extent to which the controls may be relied upon given the result of that testing [ISA 330, The Auditor's Response to Assessed Risks, paragraph 8.].