Finding unusual value in an array, list

Question

I have sales statistic data in array form to calc standard deviation or average from this data.

stats = [100, 98, 102, 100, 108, 23, 120] 

let said +-20% differential is normal situation, 23 is obviously a special case.

what's the best algorithm (in any language, pseudo or any principle) to find this unusual value?

Answer 1

You could convert them to Z-scores and look for outliers.

>>> import numpy as np
>>> stats = [100, 98, 102, 100, 108, 23, 120]
>>> mean = np.mean(stats)
>>> std = np.std(stats)
>>> stats_z = [(s - mean)/std for s in stats]
>>> np.abs(stats_z) > 2
array([False, False, False, False, False,  True, False], dtype=bool)