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A box plot shows the five-number summary. The box spans the quartiles, the line inside marks the median, and whiskers reach to the furthest point within 1.5 IQRs of the box — anything beyond is flagged as an outlier.
Interquartile range
IQR = Q₃ − Q₁
Outlier fences
Q₁ − 1.5·IQR and Q₃ + 1.5·IQR
Q₁ = 15, median = 21 and Q₃ = 27, so the IQR is 12 and the fences sit at −3 and 45.
Tukey chose it as a practical compromise: for normal data it flags only about 0.7% of observations.