First you find the mean: 34+29+39+34 = 136
136/4 = 34 The mean is 34.
Now you need to find how much each term deviates from the mean.
|34-34| = 0
|29-34| = 5
|39-34| = 5
|34-34| = 0
Now you mean/average these deviations.
(0+5+5+0)/4 = 10/4 = 2.5
The mean absolute deviation would be 2.5. :)
