The Sortino Ratio is a risk-adjusted performance measure developed by Frank A. Sortino. Unlike the Sharpe Ratio which penalizes both upside and downside volatility equally, the Sortino Ratio only considers downside deviation -- the volatility of returns that fall below a target or risk-free rate. This makes it a more refined metric for investors who are primarily concerned about losses rather than overall fluctuation.

A higher Sortino Ratio indicates better risk-adjusted performance, meaning the portfolio is generating more return per unit of harmful volatility. This is particularly useful for evaluating strategies that have asymmetric return distributions, where gains and losses are not equally distributed.

The key difference is in how risk is measured. The Sharpe Ratio uses total standard deviation (both up and down moves), while the Sortino Ratio uses only downside deviation. This means a portfolio with high upside volatility but low downside risk would have a better Sortino Ratio than Sharpe Ratio, which more accurately reflects investor experience since upside volatility is desirable.

Investors should use both metrics together for a complete picture. The Sharpe Ratio gives a broader view of risk-adjusted returns, while the Sortino Ratio focuses specifically on the risk that matters most -- the risk of losing money. When comparing funds or strategies, a consistently higher Sortino Ratio suggests the manager is better at managing downside risk.

While the Sortino Ratio provides valuable insights, it has limitations. It requires sufficient data points to accurately estimate downside deviation -- short time periods may not capture true downside risk. The ratio is also sensitive to the target return chosen; using different benchmarks can produce significantly different results.

Additionally, the Sortino Ratio assumes that downside returns follow a normal distribution, which may not hold true during extreme market events. It also does not account for liquidity risk, concentration risk, or other qualitative factors that can impact portfolio performance. Always consider the Sortino Ratio alongside other metrics and qualitative analysis when making investment decisions.