|P.Mean: Whiskers in a boxplot (created 2012-07-27).
News: Sign up for "The Monthly Mean," the newsletter that dares to call itself average, www.pmean.com/news.
Someone asked about how SPSS drew the "whiskers" in a boxplot. The length of the whiskers is supposed to be the distance from the 25th percentile to the minimum (75th percentile to the maximum) unless there are outliers. Outliers are defined as anything more than 1.5 box widths away from the end of either box.
As far as I know, SPSS follows this standard. I, personally, don't like the highlighting of individual outliers, but that's a fairly consistent norm among software programs.
Some programs like SigmaPlot implement whiskers based on a rule involving the 5th and 95th percentile or some variant, but this is not the way boxplots were originally defined.
This page was written by Steve Simon and is licensed under the Creative Commons Attribution 3.0 United States License. Need more information? I have a page with general help resources. You can also browse for pages similar to this one at Incomplete pages.