Fitch Ratings released revised non-agency MBS surveillance criteria, but most of the changes had been implemented earlier and the updated procedures for reviewing credit ratings are not expected to have a material impact on existing deals. The rating service did note that it is in the process of developing a new nonprime MBS loan loss model that likely will have a negative impact on current ratings. The new nonprime model incorporates a new regression analysis and more conservative rating stress scenarios, Fitch said. The updated surveillance criteria include an updated model for prime MBS that was released in August 2011.