  We evaluate a number of systematic effects that are important for an
experimental microwave measurement of the $n$$=$$2$ S-to-P intervals in atomic
hydrogen. The analysis is important for both re-evaluating the best existing
measurement [Lundeen and Pipkin, PRL 46, 232 (1981)] of the
2S$_{1/2}$-to-2P$_{1/2}$ Lamb shift, and for a new measurement that is ongoing
in our laboratory. This work is part of a larger program to understand the
several-standard-deviation discrepancies between various methods for
determining the proton charge radius.
