Risk of Ruin, explained.
Risk of ruin is the probability of crossing a failure boundary over a stated period. It is a property of a model, a threshold, and a time horizon.
Why it happens
Failure risk accumulates across exposures, and the size of each loss determines how much room remains for recovery. A system can have attractive ordinary outcomes yet remain vulnerable to an absorbing failure state.
Risk of ruin is the probability that a stochastic process crosses a specified absorbing boundary over a stated horizon. The probability depends on the model, starting resources, and what counts as ruin.
Look for this pattern
State exactly what counts as ruin before comparing scenarios. A probability of dropping below an operating reserve within one year cannot be compared directly with a lifetime probability of reaching zero.
A worked example
A reserve for uncertain expenses
A project defines failure as having fewer than ten resource units left, not literally exhausting every unit.
The same sequence of expenses can cross that boundary earlier than a zero-balance boundary.
Choose the boundary because it reflects when the project can no longer continue, then evaluate the stated horizon.
A common misconception
“Risk of ruin is one fixed number for a strategy.”
Starting reserves, dependence, stake sizes, stopping rules, and the horizon all change the probability.
What this explanation leaves out
- Different boundaries and time horizons answer different questions. Estimates under a simple model need not describe real-world failure probabilities.
How is risk of ruin related to gambler's ruin?
Gambler's ruin is a classic model of boundary crossing with finite resources. Risk of ruin is a broader term used for specified failure thresholds in other stochastic processes.
Which boundary represents inability to continue, and how long must the system survive?