whisker of boxplot

[This article was first published on One Tip Per Day, and kindly contributed to R-bloggers]. (You can report issue about the content on this page here)
Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.

From Wiki:
“… the bottom and top of the box are always the 25th and 75th percentile (the lower and upper quartiles, respectively), and the band near the middle of the box is always the 50th percentile (the median). But the ends of the whiskers can represent several possible alternative values…”
In R’s default boxplot{graphics} code,

upper whisker = min(max(x), Q_3 + 1.5 * IQR)
lower whisker = max(min(x), Q_1 – 1.5 * IQR)

where IQR = Q_3 – Q_1, the box length.
So the upper whisker is located at the *smaller* of the maximum x value and Q_3 + 1.5 IQR,
whereas the lower whisker is located at the *larger* of the smallest x value and Q_1 – 1.5 IQR.

The range can be adjusted via argument range in boxplot() function, whose default value is 1.5. Here is what’s described in the boxplot() document:

rangethis determines how far the plot whiskers extend out from the box. If range is positive, the whiskers extend to the most extreme data point which is no more than range times the interquartile range from the box. A value of zero causes the whiskers to extend to the data extremes.

Here is a full discussion about the whisker of boxplot in default R code:
http://r.789695.n4.nabble.com/Whiskers-on-the-default-boxplot-graphics-td2195503.html

Here is a useful plot from wikipedia for better understanding the boxplot by comparing the box plot against the probability density function (theoretical histogram) for a normal N(0,1σ2) distribution.

To leave a comment for the author, please follow the link and comment on their blog: One Tip Per Day.

R-bloggers.com offers daily e-mail updates about R news and tutorials about learning R and many other topics. Click here if you're looking to post or find an R/data-science job.
Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.

Never miss an update!
Subscribe to R-bloggers to receive
e-mails with the latest R posts.
(You will not see this message again.)

Click here to close (This popup will not appear again)