I present a new procedure to forecast the Bayes factor of a future observation by
computing the Predictive Posterior Odds Distribution (PPOD). This can assess the
power of future experiments to answer model selection questions and the probability
of the outcome, and can be helpful in the context of experiment design.
As an illustration, I consider a central quantity for our understanding of the cosmological
concordance model, namely the scalar spectral index of primordial perturbations,
nS. I show that the Planck satellite has over 90% probability of gathering
strong evidence against
n
S = 1, thus conclusively disproving a scale–invariant spectrum.
This result is robust with respect to a wide range of choices for the prior on nS.