Departure from symmetry. In statistics, the coefficient of skewness γ₁ of a random variable or of a probability distribution is defined as γ₁ = μ₃/σ³, where μ₃ is the third moment about the mean and σ is the standard deviation. Where γ₁ > 0 the typical curve trails off toward the right and hence is said to be skewed to the right; when γ₁ < 0 the longer tail is on the left, and the curve is said to be skewed to the left.