For a set of readings the lower quartile is 26 and the upper quartile is 38. A reading counts as an outlier if it falls below Q1 − 1.5 × IQR or above Q3 + 1.5 × IQR. Which one of these four readings is an outlier?

The interquartile range is 38 − 26 = 12, so 1.5 × IQR = 18 and the two fences sit at 26 − 18 = 8 and 38 + 18 = 56. Only 58 lies beyond a fence. The reading 50 is above the upper quartile but still comfortably inside the fence, and 20 and 12 are below the lower quartile yet still above 8 — lying outside the quartiles is ordinary, since half of any data set does, and only the fences mark a value as unusual.