How Many Trades Until Statistical Failure?

The Sample Size Problem

Traders often evaluate strategies after 20 or 30 trades. Statistically, this tells you almost nothing — you need at least 200 trades to achieve a 95% confidence level that your observed win rate is within ±5% of your true win rate (based on binomial confidence interval calculations). But here's the catch: the more trades you take, the more chances for a catastrophic losing streak.

Time is both your friend (law of large numbers) and your enemy (more exposure to ruin).

The Math of Losing Streaks

With a 50% win rate, the expected longest losing streak follows a logarithmic pattern:

• In 100 trades: approximately 7 consecutive losses (probability of a 7-streak: ~1 in 128)
• In 500 trades: approximately 9 consecutive losses
• In 1,000 trades: approximately 10 consecutive losses

If you risk 3% per trade, a 10-trade losing streak means a 26% drawdown. At 5% risk, it's 40%. These are not edge cases — they are statistically expected events over sufficient sample sizes.

When Does Ruin Become Inevitable?

For a strategy with:

• 45% win rate
• 1:2 risk-reward
• 3% risk per trade

After 100 trades: ~5% ruin probability

After 500 trades: ~12% ruin probability

After 2,000 trades: ~18% ruin probability

As Larry Williams, winner of the 1987 World Cup Trading Championship, observed: "every strategy has a shelf life measured in trades, not calendar days." The longer you trade, the more likely you encounter the worst-case sequence. Position sizing must account for this.

Finding Your Time Horizon

Use simulation-based modeling to test your strategy over different trade counts. If ruin probability climbs steeply with more trades, this suggests your risk per trade may be too high.

Test your numbers with the Risk of Ruin Calculator. RangeSight lets you model different timeframes with real market data.

Conceptually this provides a framework. In practice, outcomes vary across simulations. Testing multiple scenarios reveals how consistent a strategy actually is.