Sortino Ratio

The Sortino ratio is a variation of the Sharpe ratio that only considers downside volatility when measuring risk. Instead of dividing by the total standard deviation of returns (which includes both gains and losses), the Sortino ratio divides by the downside deviation — the standard deviation of negative returns only. This makes it a better measure for strategies with asymmetric return profiles, which includes most options strategies.

Why It Matters

The Sharpe ratio has a fundamental flaw for options traders: it penalizes upside volatility. If your strategy occasionally produces large gains (a long call hitting big, a crash hedge paying off), the Sharpe ratio treats that as "risk" and punishes you for it. The Sortino ratio fixes this by only counting downside moves as risk, which better reflects how traders actually think about risk.

This distinction is especially important for options strategies that have skewed return distributions. A strategy that buys OTM calls and loses small amounts most months but occasionally makes a large gain will have a mediocre Sharpe ratio but might have an excellent Sortino ratio. Conversely, a strategy that sells options and collects steady income but occasionally suffers a large loss might have a decent Sharpe but a poor Sortino — revealing the hidden downside risk.

How It Works

The formula:

Sortino Ratio = (Portfolio Return - Target Return) / Downside Deviation

The target return is often set to the risk-free rate or zero. Downside deviation is calculated using only the returns that fall below the target.

Calculating downside deviation:

1. Collect all periodic returns (daily, weekly, or monthly)
2. Identify returns that fall below the target (the "bad" returns)
3. Square each shortfall (difference between return and target)
4. Average the squared shortfalls across all periods (including zero for periods above target)
5. Take the square root

Interpreting Sortino ratios:

• Below 1.0: The strategy's returns are not adequately compensating for its downside risk
• 1.0 to 2.0: Good downside risk-adjusted performance
• Above 2.0: Excellent — strong returns with limited downside
• Above 3.0: Outstanding — very high returns relative to downside risk

Sortino vs. Sharpe comparison:

• Strategy A (credit spreads): Sharpe 1.2, Sortino 1.0 — the Sortino is lower because most of the volatility is on the downside (occasional large losses)
• Strategy B (long straddles): Sharpe 0.7, Sortino 1.8 — the Sortino is much higher because most of the volatility is on the upside (occasional large wins)

Strategy B might be the better risk-adjusted trade even though Strategy A has a higher Sharpe ratio.

When to use Sortino over Sharpe:

• Evaluating strategies with asymmetric payoffs (long options, tail hedges)
• Comparing strategies that have different return distributions
• When you care specifically about downside risk rather than total variability
• Anytime your return distribution is significantly skewed

Quick Example

Your options portfolio produces these monthly returns over 6 months: +3%, +2%, -5%, +4%, +1%, -2%. Target return is 0%.

Average monthly return: 0.5% Downside returns: -5% and -2% Downside deviation: sqrt(((5%)^2 + (2%)^2 + 0 + 0 + 0 + 0) / 6) = sqrt((0.0025 + 0.0004) / 6) = sqrt(0.000483) = 2.2%

Monthly Sortino = 0.5% / 2.2% = 0.23 Annualized Sortino = 0.23 x sqrt(12) = 0.79

Compare to Sharpe: the standard deviation of all returns is 3.1%, giving a monthly Sharpe of 0.5% / 3.1% = 0.16 and annualized Sharpe of 0.55. The Sortino is higher because it does not penalize the positive months, giving a more accurate picture of the downside risk.