Okay...Let me see if I've got it...
I'm just trying to use the default boxplot {graphics} capability in R...
So I call something like the following:
> boxplot(mpg~cyl,data=mtcars, main="Car Milage Data", xlab="Number of
> Cylinders", ylab="Miles Per Gallon") \
That produces something as shown in the following:
http://www.statmethods.net/graphs/images/boxplot1.jpg
When that default boxplot is called, i.e. boxplot {graphics}, as shown in the
line of code above, it is actually calling into boxplot.stats {grDevices}.
When boxplot.stats {grDevices} is called it has a default value for "coef" of
1.5, i.e. coef = 1.5.
If I understand the purpose of "coef" correctly, it means that the
âwhiskersâ should extend out 1.5 times the length of the box away from the
box. Is that correct?
Now I look back at the plot, and I'm not sure how 1.5 times the length of the
box corresponds with the whisker lengths shown in the image:
http://www.statmethods.net/graphs/images/boxplot1.jpg
Is it that the whisker length is a total of 1.5 the length of the box and
centered about the median (2nd Quartile)?
Just trying to get a handle on this, so thanks again for all the help in
deciphering this.
________________________________
From: RJ Cunningham <ro...@iinet.net.au>
ast.net>
Cc: R Project Help <R-help@r-project.org>
Sent: Tue, May 11, 2010 9:57:48 PM
Subject: Re: [R] Whiskers on the default boxplot {graphics}
I think not. Isn't the "secret" here?
Arguments:
x: a numeric vector for which the boxplot will be constructed
('NA's and 'NaN's are allowed and omitted).
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. A
value of zero causes the whiskers to extend to the data
extremes (and no outliers be returned).
do.conf,do.out: logicals; if 'FALSE', the 'conf' or 'out'
component respectively will be empty in the result.
Details:
The two 'hinges' are versions of the first and third quartile,...
On Wed May 12 10:35 , Jason Rupert sent:
Humm....Maybe I need to look some place else than boxplot.stats {grDevices} for
a definition of how the upper/lower whiskers are produced.
>
>>
>By any chance are they "the lowest datum still within 1.5 IQR of the lower
>quartile, and the highest datum still within 1.5 IQR of the upper quartile"?
>
>>
>None of the links from boxplot.stats {grDevices} seemed to reveal the secret
>definition of the R whiskers.
>
>>
>Thanks again.
>
>
>
>
>
>>
>----- Original Message ----
>>
>>
>To: David Winsemius <dwinsem...@comcast.net>
>>
>Cc: R Project Help <R-help@r-project.org>
>>
>Sent: Tue, May 11, 2010 9:26:25 PM
>>
>Subject: Re: [R] Whiskers on the default boxplot {graphics}
>
>>
>Wowzers...
>
>>
>From ?boxplot.stats:
>
>>
>Details
>
>>
>The two âhingesâ are versions of the first and third quartile, i.e., close
>to quantile(x, c(1,3)/4). The hinges equal the quartiles for odd n (where n <-
>length(x)) and differ for even n. Whereas the quartiles only equal
>observations for n %% 4 == 1 (n = 1 mod 4), the hinges do so additionally for
>n %% 4 == 2 (n = 2 mod 4), and are in the middle of two observations otherwise.
>
>>
>The notches (if requested) extend to +/-1.58 IQR/sqrt(n). This seems to be
>based on the same calculations as the formula with 1.57 in Chambers et al.
>(1983, p. 62), given in McGill et al. (1978, p. 16). They are based on
>asymptotic normality of the median and roughly equal sample sizes for the two
>medians being compared, and are said to be rather insensitive to the
>underlying distributions of the samples. The idea appears to be to give
>roughly a 95% confidence interval for the difference in two medians.
>
>
>
>>
>Is a notch equal to the upper/lower whisker? Is this just a difference of
>terminology or something?
>
>>
>Thanks again for all the insights.
>
>
>
>
>>
>----- Original Message ----
>>
>From: David Winsemius <dwinsem...@comcast.net>
>>
>>
>Cc: R Project Help <R-help@r-project.org>
>>
>Sent: Tue, May 11, 2010 9:00:15 PM
>>
>Subject: Re: [R] Whiskers on the default boxplot {graphics}
>
>
>>
>On May 11, 2010, at 9:45 PM, Jason Rupert wrote:
>
>>
>> How are the lower/upper whiskers defined in the default version of boxplot
>> {graphics}?
>>
>>
>>
>> I tried help(boxplot) and searching www.rseek.org, but I was unable to
>> determine an absolute answer.
>
>>
>You need to follow the links from the help pages and tin this case it appears
>that you did not follow the one to
>
>>
>?boxplot.stats
>
>>
>>
>>
>> I checked out the definition of boxplot according to Wikipedia
>> (http://en.wikipedia.org/wiki/Box_plot\), but it also had several approaches
>>
>> listed for how the whiskers could be determined, so I'm just curious how the
>> default
>>
>> boxplot {graphics} does it.
>>
>>
>>
>> Thanks for any feedback
>
>>
>Follow links with the R help system.
>
>>
>> and insights.
>
>
>
>>
>David Winsemius, MD
>>
>West Hartford, CT
>
>
>
>
>>
>______________________________________________
>R-help@r-project.org mailing list
>https://stat.ethz.ch/mailman/listinfo/r-help
>>
>PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>>
>and provide commented, minimal, self-contained, reproducible code.
>
>
>
>
>
>>
>______________________________________________
>R-help@r-project.org mailing list
>https://stat.ethz.ch/mailman/listinfo/r-help
>>
>PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
>>
>and provide commented, minimal, self-contained, reproducible code.
>
[[alternative HTML version deleted]]
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
