A skewed frequency distribution is a distribution in which the data is not spread evenly — rather, the data is clustered at one end. In this case, the mean and the median are not equal, causing the data set to be skewed. A skewed distribution is neither symmetric nor normal. In general, there are two types of skewed frequency distributions.

Skewed Distribution	Description
Skewed Left / Negatively Skewed	The distribution has a long left tail and the median is greater than the mean.
Skewed Right / Positively Skewed	The distribution has a long right tail and the median is less than the mean.

The difference between normal and skewed distributions can be visualized in the following applet. The measures of center and spread that best describe a skewed distribution are the median and the five-number summary, respectively. The median is preferred because it is less affected by outliers, while the mean will fall in the direction of the tail of the distribution.