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"The type of fallacy involved in projecting loss experience from a universe of non-insured bonds onto a deceptively-similar universe in which many bonds are insured pops up in other areas of finance. “Back-tested” models of many kinds are susceptible to this sort of error. Nevertheless, they are frequently touted in financial markets as guides to future action. (If merely looking up past financial data would tell you what the future holds, the Forbes 400 would consist of librarians.)

''Indeed, the stupefying losses in mortgage-related securities came in large part because of flawed, history-based models used by salesmen, rating agencies and investors. These parties looked at loss experience over periods when home prices rose only moderately and speculation in houses was negligible. They then made this experience a yardstick for evaluating future losses. They blissfully ignored the fact that house prices had recently skyrocketed, loan practices had deteriorated and many buyers had opted for houses they couldn’t afford. In short, universe “past” and universe “current” had very different characteristics. But lenders, government and media largely failed to recognize this all-important fact.''

''Investors should be skeptical of history-based models. Constructed by a nerdy-sounding priesthood using esoteric terms such as beta, gamma, sigma and the like, these models tend to look impressive. Too often, though, investors forget to examine the assumptions behind the symbols. Our advice: Beware of geeks bearing formulas."''


 * &mdash;Warren Buffett, To the Shareholders of Berkshire Hathaway Inc., 2009

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