Value at Risk
VaR is a quantile of the P&L distribution over a horizon — a number that says ‘we lose more than this only p percent of the time,’ until the tail arrives.
Definition
Value at Risk refers to a number that says ‘we lose more than this only p percent of the time,’ until the tail arrives. Keep that definition fixed when comparing series, managers, or regimes — renaming the same tape does not create a new signal.
Why it matters
It shows up in factor research, attribution, and capacity debates — whether a return slice is skill, style, or fee drag. When a number that says ‘we lose more than this only p percent of the time,’ until the tail arrives shifts, related hedges, limits, and narratives usually need an explicit update rather than a quiet assumption.
Case
Suppose a desk is positioned for the opposite of what value at risk is saying. If a number that says ‘we lose more than this only p percent of the time,’ until the tail arrives moves against that book, the first question is not “is the story clever?” but whether size, hedges, and stop logic still match the observation.
How to read it
Check definition stability across universes, costs, and regimes before treating a backtest as portable. Prefer a short written null hypothesis for Value at Risk: what would falsify the current reading in the next window?
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