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Confidence level and selected tail probability

At confidence c, the selected worst-tail probability is 1−c.

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Convert percentages before subtracting

A 90% confidence input is c=0.9, so 1−c=0.1 or 10%. A 97.5% input gives a selected 2.5% tail. Subtracting 90 from 1 mixes percentage numbers and decimal probability units.

Keep threshold exceedance separate

In a discrete model, probability strictly above the selected lower quantile need not equal 1−c. At 90% confidence in the original model, strictly above VaR 10 is only 5%; at or above 10 is 30%. Neither is the selected 10% mass by itself.

Read three probability questions

At 90% confidence in the original model
MeasureAmount
Selected ES tail10%
Strictly above VaR 105%
At or above VaR 1030%

Retain model uncertainty

The confidence parameter describes a quantile of the supplied model. It does not certify that historical inputs remain valid or that losses beyond the model’s observed maximum are impossible.

Further references