It's common to put stars on barplots or boxplots to show the level of significance (p-value) of one or between two groups, below are several examples:
The number of stars are defined by p-value, for example one can put 3 stars for p-value < 0.001, two stars for p-value < 0.01, and so on (although this changes from one article to the other).
And my questions: How to generate similar charts? The methods that automatically put stars based on significance level are more than welcome.
I know that this is an old question and the answer by Jens Tierling already provides one solution for the problem. But I recently created a ggplot-extension that simplifies the whole process of adding significance bars: ggsignif
Instead of tediously adding the geom_line
and geom_text
to your plot you just add a single layer geom_signif
:
library(ggplot2)
library(ggsignif)
ggplot(iris, aes(x=Species, y=Sepal.Length)) +
geom_boxplot() +
geom_signif(comparisons = list(c("versicolor", "virginica")),
map_signif_level=TRUE)
To create a more advanced plot similar to the one shown by Jens Tierling, you can do:
dat <- data.frame(Group = c("S1", "S1", "S2", "S2"),
Sub = c("A", "B", "A", "B"),
Value = c(3,5,7,8))
ggplot(dat, aes(Group, Value)) +
geom_bar(aes(fill = Sub), stat="identity", position="dodge", width=.5) +
geom_signif(stat="identity",
data=data.frame(x=c(0.875, 1.875), xend=c(1.125, 2.125),
y=c(5.8, 8.5), annotation=c("**", "NS")),
aes(x=x,xend=xend, y=y, yend=y, annotation=annotation)) +
geom_signif(comparisons=list(c("S1", "S2")), annotations="***",
y_position = 9.3, tip_length = 0, vjust=0.4) +
scale_fill_manual(values = c("grey80", "grey20"))
Full documentation of the package is available at CRAN.