Mean deviation of a series is the arithmetic average of the deviations of various items from the median or mean of that series.
Median is preferred since the sum of the deviations from the median is less than the from the mean. So the values of mean deviation calculation from median is usually less than that calculated from mean.
Mean deviation is known as first Moment of Dispersion.
Coefficient of mean deviation from mean = Mean Deviation ( M. D)/ Arithmetic Mean
Coefficient of mean deviation from median = Mean Deviation (M. D.)/ Median
Coefficient of mean deviation from mode = Mean Deviation( M. D)/ Mode