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The asymmetry of your return distribution. The shape of your payoff profile.
Skewness measures the asymmetry of a return distribution. Positive skew means a right tail (occasional large gains, frequent small losses); negative skew means a left tail (occasional large losses, frequent small gains). Equity returns typically have negative skew — crashes are sharper than rallies. Selling options has extreme negative skew.
Skew = E[(R − μ)³] / σ³Skew = E[(R − μ)3] / σ3Cube each deviation from the mean (preserving sign), average, divide by σ³. The result is unitless and interpretable directly.
Selling out-of-the-money puts gives positive skew on the underlying (you collect small premiums and occasionally pay big — but wait, that is for the BUYER). Actually: selling options gives negative skew (frequent small premiums, occasional disasters). Most "income" strategies are mathematically picking up nickels in front of a steamroller — high win rate, very negative skew.
Buying lottery tickets, venture investing, and trend-following all have positive skew. The expected value calculation looks bad but the occasional huge winner can make the strategy profitable.
S&P 500 monthly returns since 1926 have skewness of approximately −0.5 — left-skewed. Crashes are sharper than rallies.
Long-volatility ETFs (VXX) have skewness of +2.5 — extreme positive skew. Long stretches of small losses, occasional huge spikes.
"Good" skewness depends on your preference. Most investors should prefer slightly positive skew (asymmetric upside). Negatively-skewed strategies may have higher win rates but worse risk-adjusted long-run results.
Occasional large losses, frequent small gains. Looks great on a win-rate basis but can be brutal when the tail event hits.
Trend following, momentum, venture/private equity, long-volatility, lottery-ticket bets. All have low win rate but occasional huge winners.
Skewness is the third moment (asymmetry). Kurtosis is the fourth moment (fat-tailedness). They are independent — a distribution can be skewed in either direction with any kurtosis level.
Yes — in the tail risk section alongside kurtosis.
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