Outliers for ungrouped data
An outlier is any observation that is $1.5(IQR)$ away from the first quartile ($Q_1$) or third quartile ($Q_3$).
$x$ is an outlier if $x$ is below $Q_1 -1.5\times IQR$ or above $Q_3+1.5\times IQR$,
- $Q_1$ is the first quartile,
- $Q_3$ is the third quartile,
- $IQR = Q_3-Q_1$ is an inter-quartile range.