  A generalized analytical tripartite loss model is posited for Mach-Zehnder
interferometer (MZI) phase sensitivity which is valid for both arbitrary photon
input states and arbitrary system environmental states. This model is shown to
subsume the phase sensitivity models for the lossless MZI and the ground state
MZI. It can be employed to develop specialized models useful for estimating
phase sensitivities, as well as for performing associated design trade-off
analyses, for MZIs which operate in environmental regimes that are not
contained within the ground state MZI's envelope of validity. As a simple
illustration of its utility, the model is used to develop phase sensitivity
expessions for an MZI with "excited" internal arms and an MZI with "excited"
output channels. These expressions yield a conditional relationship between the
expected number of photons entering an MZI and its efficiency parameters
which-when satisfied-predicts an enhanced phase sensitivity for the MZI with
"excited" output channels relative to that for the MZI with "excited" internal
arms.
