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The Uselessness of Zagat's Cost Esti...   Short Sharp Strike
Oct 10 2007 5:09PM EDT

VaR Datapoint of the Day

Morgan Stanley has some of the most sophisticated risk-management systems on planet earth. And yet, somehow,

The company disclosed today that daily trading losses during the quarter exceeded the firm's trading value-at- risk calculation on six days during the quarter.

Needless to say, this is something which is not meant to happen. "Value at risk" means, basically, the amount of money you could conceivably lose in one day. To lose more than that in a day is a sign that your models might well be broken. To lose more than that six days in one quarter is a sign of utter cluelessness.

Update: jck has the rather misleading graphic, from Morgan Stanley's Q3. It shows four trading days with more than $125 million in losses, and three days with losses of somewhere between $50 million and $125 million, which means that Morgan Stanley's VaR was probably somewhere around $75 million or so.

What it doesn't show is just how skewed the distribution really is: on its worst day, Morgan Stanley lost $390 million. Which is more than four times its VaR. Fat tail, much?

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