What is a portfolio stress test?
A stress test estimates what would happen to a portfolio in a specific adverse scenario — a recession, a sharp rise in interest rates, a sell-off in technology stocks, a collapse of the dollar. Instead of relying on averages or statistical measures such as volatility, it asks a concrete question: if this happened, how much would the portfolio lose, and where would the loss come from?
Regulators require banks to run stress tests. For private investors, they are a useful way to check whether a portfolio's worst plausible cases are bearable.
How it works
Each holding is assigned a shock — the change in value it would suffer in the scenario — based on history or on its sensitivities. The change in the portfolio's value is the sum over all holdings:
where is the value of holding and its shock in the scenario.
Historical scenarios replay real crises: the 2008 financial crisis, the 2020 pandemic crash, the 2022 rise in interest rates. Hypothetical scenarios describe events that have not happened in exactly that form, such as a 30% fall in technology stocks combined with a 10% fall in the dollar.
A simple illustration
A €100,000 portfolio in a severe recession scenario:
| Holding | Value | Shock | Change |
|---|---|---|---|
| Global equity ETF | €50,000 | −35% | -17,500 |
| Technology stocks | €15,000 | −45% | -6,750 |
| Government bond ETF | €25,000 | +5% | +1,250 |
| Cash | €10,000 | 0% | 0 |
| Portfolio | €100,000 | -23,000 |
The portfolio would lose about 23%. Almost a third of the loss comes from the technology stocks, which make up only 15% of its value.
How to use the results
- Check against tolerance. If the loss in a plausible scenario is more than could be borne without selling, the portfolio is too risky.
- Find the weak points. The breakdown shows which holdings or exposures drive the loss.
- Compare scenarios. A portfolio can be robust in a recession and vulnerable to inflation, as balanced portfolios were in 2022.
Limits
- Shocks are assumptions. Future crises rarely repeat past ones exactly.
- Correlations change. In a crisis, assets that normally diversify each other can fall together.
- One path, not a probability. A stress test shows what a scenario would do, not how likely it is; it complements measures such as Value at Risk rather than replacing them.
Related topics
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