Skewness

What it measures

Skewness measures the asymmetry of a return distribution — whether extreme moves have tended to favor one direction over the other. A stock with positive skew has historically had occasional large up-moves punctuating a stream of small, frequent down-drifts. Negative skew is the reverse pattern: frequent small gains punctuated by occasional sharp crashes — a pattern very familiar from stock market crash dynamics.

How readings are interpreted

Positive skew: the distribution's tail stretches further to the upside — consistent with lottery-ticket-like return profiles (frequent small losses, rare large wins). Negative skew: the tail stretches further to the downside — consistent with the classic "picking up pennies in front of a steamroller" pattern where gains are steady but losses, when they come, are sharp.

Conventional levels

• Positive skewness: historical tendency toward occasional large up-moves amid more frequent small declines
• Negative skewness: historical tendency toward occasional sharp declines amid more frequent small gains
• Near-zero skewness: roughly symmetric distribution of gains and losses
• Magnitude matters as much as sign — mild skew is common and unremarkable; strong skew in either direction is the more notable signal

Where it works, and where it does not

A backward-looking statistical characterization, most useful for understanding what TYPE of return profile a security has historically exhibited, informing expectations and risk management rather than predicting the next move's direction.

Commonly read alongside

• Kurtosis: The natural companion statistic — together they describe the full shape of the distribution beyond what standard deviation alone captures
• Ulcer Index: Negatively skewed names (prone to sharp sudden drops) tend to also show up with elevated Ulcer Index readings
• Options pricing: Skew has direct, well-known analogues in options markets (volatility skew), where it's a first-class consideration

Known limitations

• Historical skew is not a guarantee of future skew — a security's return profile can and does change over time
• Easy to misread as a directional signal when it's actually a distribution-shape characterization
• Requires a meaningfully long lookback to be statistically reliable, so it's slow to reflect genuine changes in a security's behavior

In practice

• Use to set expectations: negatively skewed names warrant more attention to downside protection (stops, hedges); positively skewed names may be more tolerable to simply hold through small drawdowns
• Particularly relevant context for options strategies, where skew has direct pricing implications
• Combine with Kurtosis for the fullest picture of a security's historical return-distribution character
• A useful screening dimension for risk-averse investors specifically looking to avoid negatively-skewed, crash-prone names