(2)Those specific requirements should only apply where necessary, that is, only where institutions can neither rely on external sources of data, nor reasonably run models satisfying the requirements set out in Part Three, Title II, Chapter 3, of Regulation (EU) No 575/2013. Therefore, conditions should be laid down to ensure that there are no other sources for estimating default probabilities and losses given default, and that the cases in which institutions can neither rely on external sources, nor reasonably run their models do not represent an excessive amount of their portfolios considering the requirements laid down in Article 325bn(1) of that Regulation. Institutions should assess those conditions frequently enough to take into account potential changes, including changes to the availability of external data sources, and considering the frequency with which the own funds requirements for market risk are reported.