  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.
