Mathematically more accurate methods have been available for some time for estimating the value of mortgage servicing portfolios, but industry observers say many firms cling to older systems. Option-adjusted spread analysis has been around for about 20 years, after Wall Street MBS traders wanted a better method to predict prepayment rates, explained Thomas Rettinger, senior vice president for MSR Management at HSBC Mortgage. In a nutshell, the OAS system estimates values by generating a