[R] Quantiles in boxplot‏

[René Brinkhuis]

Hi,

Thanks for your reply.
That was exactly the piece of information I needed.

Based
 on your information I created a custom function for calculating the 
first and third quartile according to the 'boxplot logic'.
See attachment for additional details.

Kind regards,
René.

BoxplotQuartiles <- function (v) {
                       v  <- sort(v)
                       n  <- length(v)
                       ql <- n/4   ## Length of a quartile
                       p1 <- ql*1  ## Position Q1 in vector
                       p3 <- ql*3  ## Position Q3 in vector
                       f1 <- p1%%1 ## Fractional part of p1
                       f3 <- p3%%1 ## Fractional part of p3
                       if (f1 == 0.25)
                          Q1 <- v[p1 + 0.75]
                       else if (f1 == 0.50)
                          Q1 <- v[p1 + 0.50]
                       else if (f1 == 0.75)
                          Q1 <- (v[p1 + 0.25] + v[p1 + 0.25 + 1.00]) / 2
                       else if (f1 == 0.00)
                          Q1 <- (v[p1 + 0.00] + v[p1 + 0.00 + 1.00]) / 2
                       else
                          Q1 <- 'Error in calculation Q1'
                       if (f3 == 0.25)
                          Q3 <- (v[p3 - 0.25] + v[p3 - 0.25 + 1.00]) / 2
                       else if (f3 == 0.50)
                          Q3 <- v[p3 + 0.50]
                       else if (f3 == 0.75)
                          Q3 <- v[p3 + 0.25]
                       else if (f3 == 0.00)
                          Q3 <- (v[p3 - 0.00] + v[p3 - 0.00 + 1.00]) / 2
                       else
                          Q3 <- 'Error in calculation Q3'
                       return(c(Q1, Q3))
}


> Date: Fri, 13 Jan 2012 11:14:08 -0500
> Subject: Re: [R] Quantiles in boxplot‏
> From: sarah.goslee at gmail.com
> To: rene.brinkhuis at live.nl
> CC: r-help at r-project.org
> 
> The explanation is in ?boxplot.stats (as the help for boxplot states).
> 
> Details:
> 
>      The two ‘hinges’ 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.
> 
> And so on, with references.
> 
> Sarah
> 
> 2012/1/13 René Brinkhuis <rene.brinkhuis at live.nl>:
> >
> > Hi,
> >
> > I have a simple question about quartiles in R, especially how they are calculated using the boxplot.
> > Quartiles
> >  (.25 and .75) in boxplot are different from the summary function and
> > also don't match with the 9 types in the quantile function.
> > See attachment for details.
> > Can you give me the details on how the boxplot function does calculate these values?
> >
> > Cheers,
> > Rene Brinkhuis (Netherlands)
> 
> 
> 
> 
> -- 
> Sarah Goslee
> http://www.functionaldiversity.org
 		 	   		  
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