Why did Basel move from Value-at-Risk to Expected Shortfall for market risk under FRTB, and what does ES capture that VaR misses?
An advanced Risk & Compliance question — expect it in final rounds and case-heavy interviews (IB, PE, Big-4 Transaction Services).
THE SHORT ANSWER
VaR gives a threshold loss at a confidence level but says nothing about the severity beyond it — two portfolios with the same VaR can have very different tail risk, and VaR isn't sub-additive, so it can penalize diversification incorrectly. Expected Shortfall is the average loss conditional on breaching the VaR threshold, so it captures tail magnitude and is coherent (sub-additive). FRTB adopts ES (calibrated at 97.5%) plus liquidity-horizon scaling for less-liquid risk factors and a stressed calibration period. The trade-off is that ES is harder to backtest directly, so FRTB keeps VaR-based backtesting and P&L attribution tests as model-approval gates alongside the ES capital measure.
WHAT INTERVIEWERS LISTEN FOR
- ✓VaR ignores severity beyond the threshold; not sub-additive
- ✓ES = average loss in the tail, coherent
- ✓FRTB: ES at 97.5% + liquidity horizons + stressed calibration
- ✓Backtesting still uses VaR due to ES backtest difficulty
COMMON MISTAKES
- ✗Saying ES and VaR are equivalent
- ✗Not knowing ES is the conditional tail average
- ✗Unaware of FRTB context
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