it is the mean of the mean. what you do first is mind the mean of a set of numbers. lets say you have 2, 3, and 4. 2+3+4 is 9, so 9 divided by 3 is 3. now you see how far each number is away from the mean of the data. so 2=1, 3=0, 4=1. now you add up your answers to get 2 which is the MAD. MAD really is mad! 
