  Ab initio simulations of a range of interferometric experiments are used to
identify a strong dependence on multiphoton phase shifts in above-threshold
ionization. A simple rule of thumb for interaction phase shifts is derived to
explain both the conservation of photoelectron yield and its absolute
CEP-dependence. For instance, it is found that interferometric above-threshold
ionization experiments are shifted by $\pi/4$ relative to RABBIT experiments,
and that there is no RABBIT-term in a laser-assisted photoionization experiment
with odd and even harmonics. Thus, our work helps to resolve the issues of
interpretation of quantum dynamics in attosecond and free-electron laser
sciences.
