Which data set has an outlier?

For choice C, the lower half is 8, 17, 18, 20, 20, so Q1 = 18; the upper half is 21, 23, 26, 31, 39, so Q3 = 26. The interquartile range is 26 − 18 = 8, making the upper outlier cutoff 26 + 1.5(8) = 38. Since 39 is above 38, it is an outlier, so the data set in choice C has an outlier.

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