One in a billion
Jamie Dimon, the JPMorgan Chase CEO, noted in his shareholders letter that the so-called flash crash last year was a "once in three billion years" event. Of course, that's only if you believe the models.
The flash crash was when bond prices moved by around 7 standard deviations from their normal movement levels in minutes. As Dimon pointed out, the models were based on only 200 years of Treasury data. Even for a ‘once in 200 years’ event, that only provides 1 data set, assuming that data from 200 years ago is still valid.
Banks and insurers rely heavily on models and we cannot get away from that whether for setting prices, assessing business scenarios or managing risk. But we also should not forget the inherent limitations of any model. There will always be risks not covered or not considered. There will always be assumptions that may not hold up. There can always be times when models don't operate as expected or the real world takes its own path.
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