Add stars to p-value

Question

I have calculated the Anova F-Test p-value for differences in means for several variables. Now I would like to add "stars" that indicate the significance level of the p-value. I would like to have * for significance at at the 10% level, ** at the 5% level and *** at the 1% level.

My data looks like this:

structure(list(Variables = c("A", "B", "C", "D", "E"), 
               `Anova F-Test p-Value` = c(0.05, 5e-04, 0.5, 0.05, 0.01)), 
          class = "data.frame", row.names = c(NA, -5L))

Could someone help me with the code here?

Answer 1

You can build your own function. Note however that this is not the conventional star system (it's totally okay if you mention the scale somewhere though). See e.g. here.

stars.pval <- function(x){
  stars <- c("***", "**", "*", "n.s.")
  var <- c(0, 0.01, 0.05, 0.10, 1)
  i <- findInterval(x, var, left.open = T, rightmost.closed = T)
  stars[i]
}

transform(dat, stars = stars.pval(dat$`Anova F-Test p-Value`))

  Variables Anova.F.Test.p.Value stars
1         A                5e-02    **
2         B                5e-04   ***
3         C                5e-01  n.s.
4         D                5e-02    **
5         E                1e-02   ***

Answer 2

I would suggest to use cut for this

Edit: notes. Use right = FALSE to define p <= alpha as significant, use right = TRUE for p < alpha to be significant. Also changed 0 and 1 for -Inf and Inf, this often handles boundaries better in cut.

dt$stars <- cut(dt[[2]], breaks = c(-Inf, 0.01, 0.05, 0.10, Inf), 
                labels = c("***", "**", "*", "n.s."), right = FALSE)

dt

#   Variables Anova F-Test p-Value stars
# 1         A               0.0500     *
# 2         B               0.0005   ***
# 3         C               0.5000  n.s.
# 4         D               0.0500     *
# 5         E               0.0100    **

Answer 3

There is an R builtin for this:

df$stars <- symnum(df$`Anova F-Test p-Value`, 
                     symbols   = c("***","**","*",".","n.s."),
                     cutpoints = c(0,  .001,.01,.05, .1, 1),
                     corr      = FALSE
                   )
df
  Variables Anova F-Test p-Value stars
1         A                5e-02     *
2         B                5e-04   ***
3         C                5e-01  n.s.
4         D                5e-02     *
5         E                1e-02    **

Answer 4

gtools library has a stars.pval() function that takes a numeric vector of p-values and returns stars using R's standard definition