Fitch Ratings caused a stir last month when Credit Suisse dropped the firm's rating of an RMBS because it took a more critical view of the deal than two of its rivals.

But what's really notable is how rarely rating agencies give differing opinions on deals, despite rules adopted by the Securities and Exchange Commission (SEC) more than two years ago that were designed to discourage sponsors of structured finance transactions from shopping around for the most favorable ratings. The rule hasn't produced the activity that it was intended to encourage.

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