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Define loss signs before reading tail risk

In this library’s loss convention, positive numbers are losses and negative numbers are gains.

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Choose a common unit and horizon

The original model has losses −10, 0, 10 and 50 with probabilities 30%, 40%, 25% and 5%. A −10 loss means a 10-unit gain. All outcomes belong to one stated horizon and one loss unit; they are not four consecutive returns.

Sort from smallest loss to largest loss

Original same-horizon loss model
Loss unitsProbabilityCumulative probability
−1030%30%
040%70%
1025%95%
505%100%

Convert profit or return consistently

If profit is P, define loss as L=−P. If a fixed initial value V has a simple return r, the corresponding value loss under this simplified setup is −Vr. For V=1,000,000 and r=−2%, the loss is +20,000. A +1% return produces −10,000 loss. This setup omits intervening cash flows.

Keep the output sign

A model containing only gains can have a negative VaR and negative expected shortfall under this loss convention. Replacing those numbers with zero changes the measure. Label the convention instead of assuming every risk output must be positive.

Further references