Quantiles in boxplot‏

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

A more compact (though not as readable) version is afforded by stats:::fivenum.

A convenient description is (I believe) that the hinges are the medians of the bottom and top halves of the sorted observations, with the middle observation counting in both groups if n is odd).

x <- rnorm(121)
fivenum(x)

[1] -2.4596038 -0.6034689  0.1105829  0.6686026  2.2580863

median(sort(x)[1:floor((length(x)+1)/2)])

[1] -0.6034689

median(sort(x)[ceiling((length(x)+1)/2):length(x)])

[1] 0.6686026

Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com