1st, find the mean of all values. 1+2+3+4=10 10/4=2.5 The mean is 2.5.
2nd, find the distance of each value from the mean. So 2.5-1=1.5; the distance between 1 and 2.5 is 1.5. Repeat it for all of them, and you get 1.5, .5, .5, and 1.5.
3rd, you take each distance and find the mean. 1.5+.5+.5+1.5=4 4/4=1 The mean deviation is 1.