What is the g parameter in ttestBF in the BayesFactor library?

crowston

I am trying to understand the output for a two-factor Bayesian t-test using ttestBF in the BayesFactor R library. E.g.,

summarise_draws(ttestBF(formula=mpg ~ am, data=mtcars, posterior = TRUE, iterations=10000))

# A tibble: 5 × 10
 variable     mean median     sd  mad    q5   q95 rhat ess_bulk ess_tail
   <chr>        <dbl>  <dbl>   <dbl> <dbl>  <dbl>  <dbl> <dbl>    <dbl>    <dbl>
1 mu           20.7   20.7    0.906 0.897 19.3   22.2    1.00    9154.    9445.
2 beta (0 - 1) -6.47  -6.50   1.90  1.89  -9.54  -3.33   1.00    6325.    7587.
3 sig2         26.2   25.0    7.18  6.35  16.8   39.4    1.00    8040.    8876.
4 delta        -1.30  -1.30   0.418 0.418 -1.99  -0.618  1.00    6047.    7374.
5 g            14.0    1.60 444.    1.59   0.291 22.6    1.00    6968.    8035.

There's a mysterious parameter g that does not seem to be documented anywhere that I can find. It bounces around a lot, witness the high sd.

Going down the rabbit hole, I eventually found the source code for the Gibbs sampler for the test here. g is initialized to:

g = pow(beta, 2) / sig2 + 1

and makes an appearance in computing the variance for beta and the scale for sig2

varBeta = sig2 / ( sumN/4 + 1/g );

scaleSig2 = 0.5 * ( sumySq - 2.0 * mu * sumy - beta * diffy +
                         N[0] * pow(mu - beta/2, 2) + N[1] * pow(mu + beta/2, 2) );
if(!nullModel) scaleSig2 += pow(beta, 2) / g / 2

g itself seems to be an inverse gamma:

scaleg = 0.5 * ( pow(beta,2) / sig2 + rscaleSq );
g = 1 / Rf_rgamma( 0.5 * (1 + !nullModel), 1/scaleg );

I thought the answer might have something to do with this StackExchange question about the g in a JZS prior for a Bayesian t test, but I am not seeing the connection.

Any insight into what it's doing in the model would be greatly welcomed.

richarddmorey

FYI for those seeing this thread in the future, we had this discussion here: https://github.com/richarddmorey/BayesFactor/issues/174