Why John Tukey set 1.5 IQR to detect outliers instead of 1 or 2? To define outliers, why we cannot use:
Lower Limit: Q1-1xIQR
Upper Limit: Q3+1xIQR
OR
Lower Limit: Q1-2xIQR
Upper Limit: Q3+2xIQR

As I recall, Prof. Michael Starbird, in one of his lectures in the recorded series, Joy of Thinking: The Beauty and Power of Classical Mathematical Ideas, answers this question. Dr. Starbird reports having attended the very conference presentation in which Tukey introduced this test, and during which Tukey himself was asked this very question. Tukey's answer: two seems like too much and one seems like not enough.
