  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, n_S. 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 n_S.
