  We study the power spectrum of quasi-single field inflation where strong
coupling is considered. The contribution from the massive propagator can be
divided into local and non-local contributions. The local one is the leading
contribution and is power-law suppressed as a function of mass, while the
non-local contribution is exponentially suppressed in the large mass limit. For
the local contribution, it is possible to use the effective field theory
approach to study the power spectrum in the strongly coupled region of the
parameter space. For the non-local contribution, we developed a partial
effective field theory method to simplify the calculation: When there are
multiple massive propagators, one can fully compute it after integrating out
all but one massive propagator by effective field theory. The result retains
the "standard clock" signal, which is interesting for probing the expansion
history of the primordial universe and the physics of a "cosmological
collider". The error involved compared to the full calculation is power law
suppressed by the effective mass of the heavy field.
