Tukey’s Outlier Filter

John Tukey's (1915-2000) method considers observation Y an outlier if:
Y < (Q1 − 1.5 IQR) or Y > (Q3 + 1.5 IQR),
where Q1 = lower quartile, Q3 = upper quartile, and IQR = (Q3 − Q1) is
the interquartile range. Outliers distort the average and standard deviation
and make these statistics unreliable.
John Tukey, Exploratory Data Analysis, Addison-Wesley, 1977, pp. 43-44.