> -----Original Message----- > I have question concerning box plot and it's whiskers. As I > understood from the description of the boxplot() function, if > the range value is positive the plot whiskers extend out from > the box to the most extreme data points defined by the values > of the IQR times range (default 1.5). ... from the box. For a normal distribution (N(mu, sigma) the expected position of the whisker ends would be at mu+-4*0.674*sigma (that corresponds to a two-tailed 99.3% interval, if I've not lost a factor of two somewhere).
> It suggests that the > upper and lower plot whiskers should be more less the same length. > What does it mean if they are not? How it's possible? The end of each whisker is always a data point in your data set. Data can be anywhere. In small data sets (under 20 per group) the whiskers can vary quite a lot by chance; for example try set.seed(1027) y <- rnorm(150) g <- gl(10,15) boxplot(y~g) #and note group 2. In bigger data sets the quantiles are less variable and different whisker length, like the different lengths of the box parts, becomes a more reliable indicator of asymmetry. S Ellison ______________________________________________ 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.
