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The formula for calculating the position of the whiskers is

Q3+1.5IQR and Q1-1.5IQR

But doesn't that mean that the length of the whiskers should be of the same length? How are they of different lengths in some boxplots?

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Not necessarily. The whiskers actually end at the highest point within $Q3+1.5R$ and at the lowest point above $Q1-1.5R$. So for instance if $Q3+1.5R=100$ and the highest value in your sample is $90$ then the whisker will end at $90$. You must be observing something like this.

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  • $\begingroup$ Okay so the line at both whiskers end depending on the data points in our sample? $\endgroup$ Jun 18 '17 at 16:09
  • $\begingroup$ Yes but if there are points above Q3+1.5R then it will end at Q3+1.5R otherwise at the maximum value below that. That is why if there are points above Q3+1.5R and below Q1-1.5R then the length of the whiskers look the same. $\endgroup$ Jun 18 '17 at 16:20

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