  This paper presents a protocol, or design, for the analysis of a comparative
effectiveness evaluation of abiraterone acetate against enzalutamide, two drugs
given to prostate cancer patients. The design explicitly make use of
differences in prescription practices across 21 Swedish county councils for the
estimation of the two drugs comparative effectiveness on overall mortality,
pain and skeleton related events. The design requires that the county factor:
(1) affects the probability to be treated (i.e. being prescribed abiraterone
acetate instead of enzalutamide) but (2) is not otherwise correlated with the
outcome. The fist assumption is validated in the data. The latter assumption
may be untenable and also not possible to formally test. However, the validity
of this assumption is evaluated in a sensitivity analysis, where data on the
two morbidity outcomes (i.e. pain and skeleton related events) observed before
prescription date are used. We find that the county factor does \emph{not}
explain these two pre-measured outcomes. The implication is that we cannot
reject the validity of the design.
