System Quality Number (SQN): The Metric Prop Traders Ignore

Most prop firm traders obsess over win rate. Some track profit factor. Almost none calculate System Quality Number (SQN) -- the metric that actually tells you whether your system is good enough to survive a prop firm evaluation.

SQN was developed by Dr. Van K. Tharp as a way to measure the quality of a trading system independent of position sizing. It answers a simple question: is your edge real, or are you getting lucky?

What Is SQN?

System Quality Number measures the statistical quality of your trading system. It combines your average trade result (expectancy) with the consistency of those results (standard deviation).

Formula: SQN = (Average R-Multiple / Standard Deviation of R-Multiples) x Square Root of N

Where:

Simplified Version

If you do not want to calculate R-multiples:

SQN = (Mean P&L per trade / Std Dev of P&L per trade) x sqrt(min(N, 100))

This approximation works when your risk per trade is roughly consistent.

SQN Benchmarks

Dr. Tharp's original scale:

For prop firm evaluations, you need an SQN of at least 2.0. Below 2.0, the variance in your trade results is too high relative to your edge. A losing streak of 4-5 trades (which is normal) will consume most of your drawdown buffer.

Why SQN Matters More Than Win Rate

Win rate tells you how often you win. Profit factor tells you the ratio of wins to losses. Neither tells you how consistently your system performs relative to its edge.

Consider two systems:

System A:

System B:

System B has a lower win rate but a slightly higher SQN because its per-trade expectancy is higher and the consistency is comparable. Over 100 trades, System B produces more profit with similar drawdown characteristics.

Now imagine the standard deviation doubles for System A (inconsistent results): SQN drops to 0.65 -- untradeable. The edge exists on paper but the variance makes it impossible to survive a prop firm's drawdown limits.

How to Calculate Your SQN

Step 1: Convert All Trades to R-Multiples

For each trade, divide your actual P&L by your initial risk (the distance from entry to stop loss in dollars).

Step 2: Calculate Mean and Standard Deviation

Mean of R-multiples: (2.0 + (-1.2) + 2.5) / 3 = 1.10

Standard deviation: Use a spreadsheet or calculator. In this example, approximately 1.93.

Step 3: Apply the Formula

SQN = (1.10 / 1.93) x sqrt(min(3, 100)) = 0.57 x 1.73 = 0.99

With only 3 trades, the SQN is not meaningful. You need at least 30 trades for a preliminary reading and 100+ for a reliable one.

SQN and Prop Firm Survival

The relationship between SQN and prop firm drawdown is direct:

This is why SQN matters specifically for prop firm traders: the drawdown constraints are fixed by the firm. You need a system with enough quality (SQN) to survive the inevitable losing streaks within those constraints.

How to Improve Your SQN

Increase Your Mean R-Multiple

The numerator of SQN is your average R-multiple. Improve it by:

Decrease Your Standard Deviation

The denominator is the consistency of results. Reduce variance by:

Increase Sample Size

SQN scales with sqrt(N), capped at 100. More trades improve SQN, but only if the new trades maintain the same mean and standard deviation. Adding marginal trades that lower your average will decrease SQN even though N increases.

Tracking SQN Over Time

Calculate your SQN on a rolling 30-trade and 100-trade basis. If the 30-trade SQN drops below 1.5, your system may be losing edge in current market conditions. Consider:

Vigil tracks R-multiples and SQN automatically from your trade data, alerting you when system quality deteriorates before it shows up as drawdown.

Calculate your SQN with a free Vigil audit. Upload 30+ trades and see your system quality score across different timeframes.