Stable distributions, also known as Lévy stable distributions, offer a powerful alternative to the normal distribution in financial modeling, particularly when dealing with asset returns. Their defining characteristic is *stability*, meaning that a linear combination of independent random variables drawn from a stable distribution also follows a stable distribution (with the same stability parameter). This property is crucial for modeling portfolio returns, as it ensures consistency across aggregation. Unlike the normal distribution, stable distributions can capture the “fat tails” and skewness often observed in financial data, reflecting the higher probability of extreme events (market crashes, unexpected booms) than predicted by a normal distribution.
The stability parameter, α (0 < α ≤ 2), dictates the tail heaviness. A lower α corresponds to heavier tails and a greater probability of extreme events. When α = 2, the stable distribution reduces to the normal distribution. The parameter β (-1 ≤ β ≤ 1) governs skewness, with β = 0 indicating symmetry, β > 0 indicating right skew (longer right tail), and β < 0 indicating left skew (longer left tail). Two additional parameters, scale (σ) and location (μ), determine the spread and center of the distribution, respectively, similar to the standard deviation and mean in a normal distribution.
The relevance to finance stems from the limitations of the normal distribution. Many financial assets exhibit non-normal return distributions. Extreme events, often driven by factors outside of standard risk models, occur more frequently than predicted by a normal curve. Using a normal distribution can thus underestimate risk, leading to inadequate risk management strategies. Stable distributions, with their ability to accommodate fat tails, offer a more realistic representation of these risks.
For example, in portfolio optimization, using stable distributions allows for constructing portfolios that are more robust to extreme market movements. Value at Risk (VaR) and Expected Shortfall (ES) calculations based on stable distributions provide a more accurate estimate of potential losses compared to calculations based on normality assumptions. Option pricing can also benefit; stable distributions can be incorporated into models to account for the higher volatility and extreme price jumps often observed in options markets.
However, stable distributions also present challenges. They generally lack closed-form expressions for their probability density function and cumulative distribution function, except in a few special cases (normal, Cauchy, and Lévy distributions). Parameter estimation can be computationally intensive and sensitive to data quality. Furthermore, the infinite variance of stable distributions when α < 2 raises theoretical concerns about traditional statistical measures based on variance. Despite these challenges, the ability of stable distributions to capture stylized facts of financial data makes them a valuable tool for risk management, portfolio optimization, and derivative pricing, particularly when dealing with assets exhibiting significant tail risk.