Dear R users, 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). 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? I'm using default value of the range. Would be garateful for any hint. Kind regards, Barbara [[alternative HTML version deleted]]
The whiskers will not be symmetrical if 1.5*IQR extends beyond the maximum or minimum values. In that case, the whisker stops at the maximum or minimum. ------------------------------------- David L Carlson Associate Professor of Anthropology Texas A&M University College Station, TX 77840-4352 ----- Original Message ----- From: "Barbara Uszczynska" <uszczynska at gmail.com> To: r-help at r-project.org Sent: Friday, July 13, 2012 7:40:30 AM Subject: [R] box plot and plot whiskers Dear R users, 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). 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? I'm using default value of the range. Would be garateful for any hint. Kind regards, Barbara [[alternative HTML version deleted]] ______________________________________________ R-help at 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.
> -----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