(25)The global financial crisis of 2008-2009 has revealed that, in some cases, institutions have also used the IRB Approach on portfolios unsuitable for modelling due to insufficient data, which had detrimental consequences for the reliability of the results. It is therefore appropriate not to oblige institutions to use the IRB Approach for all of their exposures and to apply the roll-out requirement at the level of exposure classes. It is also appropriate to restrict the use of the IRB Approach for exposure classes where robust modelling is more difficult in order to increase the comparability and robustness of own funds requirements for credit risk under the IRB Approach.