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Why are data that are one and a half times the interquartile range (1.5IQR) or more, measured from the first and third quartiles, classed as outliers. Why 1.5?

gung - Reinstate Monica
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randusr836
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    This might help: http://stats.stackexchange.com/questions/73324/when-finding-outliers-from-the-interquartile-range-why-i-have-to-multiply-by-1-5 – ilanman Oct 05 '16 at 11:23

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Maybe not the most enlightening explanation but at lease there is someone to blame...

Why one and a half times the width of the box? Why does that particular value demark the difference between "acceptable" and "unacceptable" values? Because, when John Tukey was inventing the box-and-whisker plot in 1977 to display these values, he picked 1.5×IQR as the demarkation line for outliers. This has worked well, so we've continued using that value ever since. source

and if you google for "1.5×iqr rule" there are more examples of why 1.5 actually works well, with wikihow beeing probably the best:

How to Calculate Outliers

blazej
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