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If I have a set of data and I want to display it as a boxplot, when there are some outliers in the boxplot, we indicate outliers with a *. If there are two outliers having the same value, how to put that in the boxplot? Is it still *?

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Could you be a little more specific? Are you asking about the input data to a boxplot (where you could of course have duplicate values) or about the output values a boxplot function may report? The last will probably depend on your software. For instance, the boxplot() function in R will report multiple identical outliers separately: boxplot(c(rnorm(100,0,1),5,5))$out yields two separate outliers of value 5. – Stephan Kolassa Nov 16 '12 at 8:52
There are several possibilities. Some programs - especially older ones using test-based display - will use a '2', '3' etc to indicate how many values are there. Other programs will show two stars (or circles or whatever) together. – Glen_b Nov 16 '12 at 9:35
Are you asking about doing boxplots by hand or about how some software does this? As @Glen_b said, different software will do different things. – Peter Flom Nov 16 '12 at 11:51
@StephanKolassa: I think the point is that they are plotted over the top of each other, so you can't see that there's two. – naught101 Nov 22 '12 at 5:56

I'll use R in proposing a solution. Let's simulate some data:

foo <- c(rnorm(100,0,1),5,5,5,7,7)

I see two possibilities. The first one would be to plot the boxplot and add sunflowerplots of the outliers:

bar <- boxplot(foo)

sunflowerplotted outliers

The second possibility is to plot the boxplot (which creates a single point for each outlier) and add additional points for additional outliers - which are jittered horizontally (edited as per @chl's excellent suggestion):

bar <- boxplot(foo,plot=FALSE)
points(jitter(rep(1, length(bar$out))), bar$out)

jittered outliers

Note that the first solution requires that your data are integers (otherwise you will run afoul of floating-point arithmetic, see question 7.31 in the R FAQ - in this case you will need to do some additional work to ensure R knows which floating point numbers should be treated as "equal").

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(+1) I would say points(jitter(rep(1, length(bar$out))), bar$out) is shorter :-) – chl Nov 22 '12 at 10:25
@chl: yes, that would be shorter... but then we would have n+1 points where only n should be for n outliers with the same value: one from the original boxplot and n from overplotting the jittered points. This is why I used the cumbersome table() stuff. (It would be easier if there were a way to tell boxplot() not to plot the outliers.) Or am I mistaken? – Stephan Kolassa Nov 22 '12 at 10:29
Yes sorry, I wrote quickly: check the outline= argument to boxplot(). (You'll need to manage the y-axis limits, though.) – chl Nov 22 '12 at 10:33
@chl: thank you, that definitely helps! I edited the original proposal. – Stephan Kolassa Nov 22 '12 at 10:41

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