Gavin Simpson wrote:
On Mon, 2008-08-04 at 17:00 +1000, Simon Blomberg wrote:
See ?fivenum in the stats package. If you just type
stats::fivenum
you will get the code. The crucial calculations are in the last few
lines.
That will only give the code to calculate the five number summary, but
Fernando wants to know how the confidence interval is calculated in
boxplot.stats.
To see the code just type boxplot.stats followed by return at the
command line in R.
The relevant line is:
conf <- if (do.conf)
stats[3] + c(-1.58, 1.58) * iqr/sqrt(n)
Which is working on the median (stats[3]). Details of this computation
are in ?boxplot.stats which should have been Fernando's first port of
call. Two works are cited in support of the calculation with full
references in the References section of that help page.
HTH
G
I wonder why we don't just use the exact nonparametric confidence
interval for the median, which is just as easy to compute. Also, it
will be asymmetric if the data are skewed, as it should be.
Frank
Simon.
On Mon, 2008-08-04 at 16:19 +0930, Fernando Marmolejo Ramos wrote:
Dear people
I've learnt that by using the "boxplot.stats" command in the "grDevices" library
I can get the 5-number summaries of a boxplot, plus other important information,
like the confidence interval around the median.
I'm interested in knowing the actual formula to used in that package to
calculate that confidence interval.
Can someone help me with this?
Cheers,
Fernando
--
Frank E Harrell Jr Professor and Chair School of Medicine
Department of Biostatistics Vanderbilt University
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