Research Design and Statistical Consulting
George M. Diekhoff, Ph.D.

Why Are Skewed Distributions Called “Positive” and “Negative?”

I guess my intro stat teacher taught me something because I was surprised to learn that so many people don’t know the answer to this question. This is a short post because the answer is short: It’s because the measures that are used to measure asymmetry of a distribution produce a value that’s positive when the distribution is positively skewed, and negative when the distribution is negatively skewed. Nothing fancier than that! For instance, the simplest possible measure of skewness is: 

Skewness = Mean – Median 

If the distribution is symmetrical (i.e., no skew) the measure will be 0. If the distribution is positively skewed, the measure will take on a positive value. And if the distribution is negatively skewed, the measure of skewness will take on a negative value.