# [R] Great looking plot - but what does it mean?

Earl F. Glynn efg at stowers-institute.org
Mon Jan 7 17:19:45 CET 2008

```"mika03" <someone29_7 at yahoo.de> wrote in message
news:14668788.post at talk.nabble.com...
>
> http://www.nabble.com/file/p14668788/paragraphs.png
>
> R is is world full of wonders... I created the attached plot, and I think
> it's exactly what I need! Well, actually I think it is more that wht I
> need...
>
> I wanted R to show the mean values of the categories on the x-axis and
> maybe
> the standard derivation as well.
>
> I am pretty confident that the bold horrizontal lines in the plot show the
> mean values. But what are the white boxes and the dashed lines? And what's
> that one small circle on the "Section" column supposed to mean.
>
> And if I would like to get rid of that small circle, how can I?

http://en.wikipedia.org/wiki/Box_plot

"mean" and "standard deviation" are terms that explain normally distributed
data.  A box plot is intended to display similar information but without any
assumption about the distribution of the data.

The horizontal lines are the medians (http://en.wikipedia.org/wiki/Median),
not the means.  The rectangular boxes show the middle 50% of your data --
the interquartile range (http://en.wikipedia.org/wiki/Interquartile_range).
If the median is not in the "middle" of the box , your data are skewed.

The small circle is an "outlier" (http://en.wikipedia.org/wiki/Outlier).

Many options for a boxplot are actually buried in ?bxp instead of  ?boxplot.
To suppress the outlier, look in ?bxp and note that outpch=NA should get rid
of the outlier points.

efg

Earl F. Glynn
Stowers Institute for Medical Research

```