The mean deviation (also called the mean absolute deviation) is the mean of the absolute deviations of a set of data about the data's mean.For a sample size, the mean deviation is defined by. (1). where is the mean of the distribution. The mean deviation of a list of numbers is implemented in the Wolfram Language as.he mean deviation (also called the mean absolute deviation) is the mean of the absolute deviations of a set of data about the data's mean. For a sample size, the mean deviation is defined by.