The aging of the subprime and prime mortgages that back the shrinking universe of non-agency MBS is gradually changing the performance trends of these loans, according to analysts speaking at a Fitch Ratings conference in New York this week. Selection bias – changes in the composition of the remaining subprime and prime mortgage pools as borrowers default or refinance – will mean different things for different asset classes, but differences between the two will likely become less pronounced over the next year, analysts said. Grant Bailey, a managing director at Fitch, explained that in many ways...