  Cosmic acceleration may be due to modifications of cosmic gravity and to test
this we need robust connections between theory and observations. However, in a
model independent approach like effective field theory or a broad class like
Horndeski gravity, several free functions of time enter the theory. We show
that simple parametrizations of these functions are unlikely to be successful;
in particular the approximation $\alpha_i(t)\propto\Omega_{\rm de}(t)$
drastically misestimates the observables. This holds even in simple modified
gravity theories like $f(R)$. Indeed, oversimplified approximations to the
property functions $\alpha_i(t)$ can even miss the signature of modified
gravity. We also consider the question of consistency relations and the role of
tensor (gravitational wave) perturbations.
