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Boxplot Whiskers

ylee

Occasional Contributor

Joined:

Nov 16, 2016

Hello.

 

By plotting Variability Chart and choosing "Show Box Plot", I believe the given boxplot is an Outlier Box Plot as described here.

http://www.jmp.com/support/help/Quantiles.shtml

 

Need to be sure of my understanding on this part below:

Each box has lines, sometimes called whiskers, that extend from each end. The whiskers extend from the ends of the box to the outermost data point that falls within the distances computed as follows:

3rd quartile + 1.5*(interquartile range)
1st quartile - 1.5*(interquartile range)

If the data points do not reach the computed ranges, then the whiskers are determined by the upper and lower data point values (not including outliers).

 

If I have the Q1 and Q3 as below with the top and bottom whiskers calculated:

Q1 - 1.5*IQR = -12.32811832 - (1.5*(-10.19507933 - (-12.32811832))) = -15.5276768
Q3 + 1.5*IQR = -10.19507933 + (1.5*(-10.19507933 - (-12.32811832))) = -6.99552085

But if my maximum data range is only -7.5 (and it is not an outlier), then the whisker should extend till -7.5 instead of -6.99552085, which I believe the intention for JMP is no point to report a whisker that's further that the maximum data range.

Correct?

1 REPLY
txnelson

Super User

Joined:

Jun 22, 2012

In your display window, do a shift/?, then go to the graph in question and click on it. You will be taken to documentation on the graph
Jim