[R] Boxplot has only one whisker

[David Winsemius]

On Oct 13, 2010, at 5:06 AM, Ivan Calandra wrote:

> Well, you don't use the same data for both.
> Type 2 and 7 give different values for the 25 and 75%, which  
> correspond more or less to the box hinges.
> If you take a look at how the whiskers are defined (look at ? 
> boxplot.stats):
> "|coef| this determines how far the plot ‘whiskers’ extend out from  
> the box. If |coef| is positive, the whiskers extend to the most  
> extreme data point which is no more than |coef| times the length of  
> the box away from the box."
> In your example, the box for "method1" is short; the 100% point is  
> then further that 1.5*length of the box (IQR), which is then  
> considered as outlier. Note that the default for coef is 1.5 (hence  
> the 1.5*IQR). On the other hand, for method 2, the box is longer;  
> the largest point is therefore within 1.5*IQR and plotted with the  
> upper whisker.
> Understand what I mean?
> See that post too, they might have better explanations:
> http://finzi.psych.upenn.edu/Rhelp10/2010-May/238597.html
>
> But don't forget my second comment: you boxplot() summary data. From  
> my understanding, you should not do that. Boxplot() returns the  
> statistics itself (look at the value section from ?boxplot). Try:
> bx <- boxplot(d,ylab = "Beispiel 1",range = 1.5)
> bx$stats
> [,1] [,2]
> [1,] 2.7000 2.700
> [2,] 3.0000 2.900
> [3,] 3.3000 3.300
> [4,] 4.8625 6.225
> [5,] 4.8625 8.950
>
> You can see that boxplot() recomputed the summary stats from the  
> summary stats you gave as input, and plotted them!
> You should provide raw data to boxplot(), not summary stats.

Agreed,  boxplot is doing the summarization, but it then hands the  
summaries on to bxp() to do the plotting. So if the OP wants to add  
back in a whisker or a dot or to use summaries from another source,  
then he should first look at the Value section of the boxplot help  
page for the correct list structure, and then send that list to bxp().  
The bxp help page also is where one would look to tweak various  
plotting parameters, since those are better explained there.

-- 
David.

> If you want to input summary stats, there was a post some time ago  
> on that:
> http://finzi.psych.upenn.edu/Rhelp10/2010-September/251674.html
>
> HTH,
> Ivan
>
> Le 10/13/2010 10:33, tom a écrit :
>>
>> Ivan Calandra wrote:
>>>   Try with more data points?!
>>> You have only five points, the last one being considered as outlier.
>>> Note that boxplot() requires a numeric vector for specifying data  
>>> from
>>> which the boxplots are to be produced!
>>>
>> But why is only one of the boxplots missing his whisker? I use the  
>> same data
>> for both boxplots.
>> thx,
>> tom
>>
>>
>>
>>
>
> -- 
> Ivan CALANDRA
> PhD Student
> University of Hamburg
> Biozentrum Grindel und Zoologisches Museum
> Abt. Säugetiere
> Martin-Luther-King-Platz 3
> D-20146 Hamburg, GERMANY
> +49(0)40 42838 6231
> ivan.calandra at uni-hamburg.de
>
> **********
> http://www.for771.uni-bonn.de
> http://webapp5.rrz.uni-hamburg.de/mammals/eng/mitarbeiter.php
>
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David Winsemius, MD
West Hartford, CT