(15)To ensure a correct calculation of the own funds requirements, competent authorities, when they assess the overall quality of the data, should assess whether the approach employed by the institution to proxy time series is sound. The assessment methodology should therefore verify that the requirements set out in Regulation (EU) No 575/2013 governing the usage of proxies are complied with. Where relevant, the rules laid down in that assessment methodology should differ depending on whether the time series for which a proxy has been used relates to a risk factor that passed the modellability assessment, or to a risk factor that did not.