I want to draw a boxplot with whiskers where a lower bound is equal to $Q1 - 1.5 IQR$ and an upper bound is equal to $Q3 + 1.5 IQR$, where $Q1$ is a lower quantile, $Q3$ is an upper quantile and $IQR$ ia an interquantile range.

My observations are positive, but the value $Q1 - 1.5 IQR$ is below $0$. My question is:

Where should I put a lower bound of whiskers on a boxplot: in the point $Q1 - 1.5 IQR$ (which is negative) or in the point equal to $0$, because all my observations are positive?

  • $\begingroup$ What do you by mean lower bound? The $ 1.5 \times IQR$ is a rule of thumb to highlight potential outliers. $\endgroup$ – Karl Oct 30 '18 at 13:50
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    $\begingroup$ I suggest using the lowest data value if none are outliers. $\endgroup$ – Karl Oct 30 '18 at 13:53

The whiskers should extend to the most extreme data points satisfying the criterion that they lie within $Q1-1.5IQR$ and $Q3+1.5IQR$ from the respective box edges.

e.g. see here and read about 'range'

  • $\begingroup$ Thank you for your response. $\endgroup$ – MathMen Oct 30 '18 at 14:11

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