What is "traditional yield spread analysis" and under what conditions will this approach to comparing fixed income security values be invalid or unavailable. How does option adjusted spread analysis correct for these "failures" of traditional yield spread analysis in the valuation of bonds with embedded option features? Once the option adjusted spread has been determined, how can the cost of option be calculated? What are some important pitfalls of using option adjusted spread analysis to value mortgage backed securities and other collateralized debt obligations?