Place the values in ascending order first:
4, 10, 10, 11, 12, 12, 12, 12, 13, 13, 15, 15, 17, 18, 19, 20
There are 16 values
IQR is 3/4th term - 1/4th term
The 12th value is 15 while the 4th value is 11
Therefore IQR = 15-11 which is 4
I would say the outlier is 4.
Justification: Figure out the mean with the value of 4 then discount it to see whether the mean is affected by the outlier.
