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A value $x$ is an outlier IFF

1. $x < Q_1- 1.5(IQR)$

or

2. $x > Q_3 + 1.5(IQR)$

where $Q_1$ is the lower quartile, $Q_3$ is the upper quartile, and $IQR = Q_3 - Q_1$, the interquartile range.