Risk of ruin is the probability that an account will fall to a level from which it cannot recover under a specified position sizing rule.
Risk of ruin is the probability that an account will fall to a level from which it cannot recover under a specified position sizing rule.
The idea links trading outcomes to capital preservation. A strategy can have a positive expected outcome and still face a meaningful chance of ruin if position sizes are too large relative to losses, volatility, or drawdowns. In practice, risk of ruin asks a path dependent question. It is not only whether a strategy is profitable on average. It is whether the sequence of gains and losses can drive equity down to a level where the sizing rule or the account constraints prevent recovery.
Position sizing is central to that mechanism. When size is tied to current equity, losses reduce the capital base used for future trades. That creates a compounding effect. A drawdown then does two things at once. It lowers account equity, and it lowers the absolute amount of capital that can participate in any later recovery. If losses are large enough, the account may reach a floor defined by the trader, the broker, or the practical limits of the strategy. That is the ruin threshold.
This differs from a simple drawdown statistic. Drawdown measures how far equity has fallen from a previous peak. Risk of ruin measures the probability of hitting a terminal or unacceptable capital level at any point along the path. A system can show tolerable average drawdowns and still carry a nontrivial ruin probability if adverse sequences are severe enough under the chosen sizing rule.
The concept can be defined and explained without detailed formula derivations from the sources.
The deflated Sharpe ratio is presented as a way to account for selection bias and multiple testing when evaluating whether an observed Sharpe ratio is likely to be statistically meaningful rather than a product of backtest overfitting. That makes it complementary to risk of ruin rather than a substitute for it. Risk of ruin focuses on capital survival under a sizing rule. The deflated Sharpe ratio focuses on the credibility of a reported performance statistic after accounting for the research process. Used together, they address different failure modes. One concerns account depletion. The other concerns false discovery.
The same logic applies to backtest overfitting audits. Sonar frames overfitting analysis as a check on whether a strategy result is robust or whether it may have been tuned to noise. That matters for risk of ruin because an overfit strategy can appear stable in sample while carrying much worse realized path risk out of sample. A sizing rule built on overstated edge can therefore understate ruin risk.
In plain terms, risk of ruin is a survival metric. It asks whether capital can endure the loss path implied by a strategy and a sizing rule. The more aggressive the sizing, the more sensitive the account becomes to adverse sequences. The less reliable the underlying strategy evidence, the less confidence one should have in any estimate of ruin probability.
Covered in depth in the Strategy research fundamentals pillar hub.