Bear Stearns has developed a new index of mortgage performance, in an effort to measure default rates more completely for holders of mortgage-backed securities. Gyan Sinha, head of Bear Stearns’ asset-backed research division, said the research is intended to provide an “implied” default rate from typical bad-loan characteristics. Currently, defaults are based on actual losses recorded by the loan servicer. Sinha said the actual rate might have been masked by rising house prices, since only…