THERE AREN'T MANY widely told anecdotes about the current financial crisis, at least not yet, but there's one that made the rounds in 2007, back when the big investment banks were first starting to write down billions of dollars in mortgage-backed derivatives and other so-called toxic securities. This was well before Bear Stearns collapsed, before Fannie Mae and Freddie Mac were taken over by the federal government, before Lehman fell and Merrill Lynch was sold and A.I.G. saved, before the $700 billion bailout bill was rushed into law. Before, that is, it became obvious that the risks taken by the largest banks and investment firms in the United States - and, indeed, in much of the Western world - were so excessive and foolhardy that they threatened to bring down the financial system itself. On the contrary: this was back when the major investment firms were still assuring investors that all was well, these little speed bumps notwithstanding - assurances based, in part, on their fantastically complex mathematical models for measuring the risk in their various portfolios. There are many such models, but by far the most widely used is called VaR - Value at Risk.