Precision of Inflaton Potential Reconstruction from CMB Using the   General Slow-Roll Approximation
  Through a principal component analysis, we study how accurately CMB observables can constrain inflaton potentials in a model independent manner. We apply the general slow-roll approximation in our analysis where we allow, in contrast to the standard slow-roll approximation, the possibility of variations in $V''(\phi)$ and take into account the fact that horizon crossing is not an instantaneous event. Our analysis provides a set of modes to be used in fitting observables. We find that of order five of these modes will be constrained by future observations, so a fully general data analysis package could use the amplitudes of just a handful of modes as free parameters and retain all relevant information in the data. 
Kenji Kadota, Scott Dodelson, Wayne Hu and Ewan D. Stewart