(6)Competent authorities are required to verify compliance of institutions with the specific regulatory requirements for the use of the IRB Approach, as well as evaluate the overall quality of the solutions, systems and approaches implemented by an institution, and request constant improvements and adaptations to changed circumstances in order to achieve continuous compliance with those requirements. Such an assessment requires, to a large extent, that the competent authorities exercise their discretion. Rules for the assessment methodology should, on the one hand, allow the competent authorities to exercise their discretion by carrying out additional checks to those specified in this Regulation, as necessary, and should, on the other hand, ensure harmonisation and comparability of supervisory practices across different jurisdictions. For the same reasons, competent authorities should have the necessary flexibility to apply the most appropriate optional method or any other method necessary for verifying particular requirements, having regard, among others, to the materiality of the types of exposures covered by each rating system, the complexity of the models, the particularities of the situation, the specific solution implemented by the institution, the quality of evidence provided by the institution, the resources available to the competent authorities themselves. Furthermore, for the same reasons competent authorities should be able to carry out additional tests and verifications necessary in case of doubts regarding the fulfilment of the requirements of the IRB Approach in accordance with the principle of proportionality, having regard to the nature, size and complexity of an institution’s business and structure.