  We use model selection forecasting to assess the ability of the Planck
satellite to make a positive detection of spectral index running. We simulate
Planck data for a range of assumed cosmological parameter values, and carry out
a three-way Bayesian model comparison of a Harrison-Zel'dovich model, a
power-law model, and a model including running. We find that Planck will be
able to strongly support running only if its true value satisfies |dn/d ln k| >
0.02.
