  The consistency relation for the 3-point function of the CMB is a very
powerful observational signature which is believed to be true for every
inflationary model in which there is only one dynamical degree of freedom. Its
importance relies on the fact that deviations from it might be detected in next
generation experiments, allowing us to rule out all single field inflationary
models. After making more precise the already existing proof of the consistency
relation, we use a recently developed effective field theory for inflationary
perturbations to provide an alternative and very explicit proof valid at
leading non trivial order in slow roll parameters.
