Since the beginning of the recession, Mr. Buffett has taken on a part-time and unofficial role as the United States economy’s éminence grise. He also served on President Obama’s transition team. In his letter, though, he lambasted the decisions and habits that led to the credit crisis.

In reviewing the performance of Clayton Homes, a Berkshire Hathaway subsidiary that sells manufactured homes, he pointed out that its lending arm had managed to keep foreclosure rates to less than 4 percent, even among subprime borrowers, or those with weak credit ratings.

He contrasted that relative success with the failures of just about everyone else in the same business.

“The stupefying losses in mortgage-related securities came in large part because of flawed, history-based models used by salesmen, rating agencies and investors,” he wrote.

He went on: “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.”

Also blissfully ignored, he wrote, were the perils of relying on mathematical models devised without worst-case situations in mind. Too often, he wrote, Americans have been enamored of “a nerdy-sounding priesthood, using esoteric terms such as beta, gamma, sigma and the like.”

Some skepticism about these models is overdue, he added. “Our advice: Beware of geeks bearing formulas.”

Mr. Buffett was just as scathing on the subject of derivatives, which he had likened to weapons of mass destruction long before they started eviscerating the balance sheets of banks around the world.

In his letter, Mr. Buffett explained that the danger of derivatives was not merely the difficulty in assessing their value; rather, it was the “web of mutual dependence” they create among financial institutions. Derivatives contracts keep various parties entangled for years, which, as he vividly explained, can create real hazards once those assets start deteriorating.

“Participants seeking to dodge troubles face the same problem as someone seeking to avoid venereal disease,” he wrote. “It’s not just whom you sleep with, but also whom they are sleeping with.”