Inverse function of Bayes factor for t test

bf_inv.R

## Given a (scaled Cauchy) Bayes factor for the null against the 
## alternative (Rouder et al 2009), yields the t statistic
## that would yield it. The ... arguments are passed to 
## the ttest.tstat function.
bf.inv = Vectorize(function(b10, ...){
  fn = Vectorize(function(t,...){
    BayesFactor::ttest.tstat(t,...)[["bf"]]
  }, "t")
  t0 = optimize(function(t0, ...){
    t = sqrt(t0 / (1 - t0))
    b = fn(t, ...)
    (b10 - exp(b))^2
  }, c(0,1), ...)$minimum
  sqrt(t0 / (1 - t0))
},"b10")
 

q = bf.inv(1/3, n1 = 20, rscale = sqrt(2)/2)
## Test it (should be 1/3)
BayesFactor::ttest.tstat(q, n1 = 20, rscale = sqrt(2)/2, simple = TRUE)

## Make a plot
b = seq(.05,20, len = 50)
plot(b, bf.inv(b, n1 = 20), ty = 'l', ylab = "t statistic",xlab = "Bayes factor", log = "x")