  For settings with a binary treatment and a binary outcome, instrumental
variables can be used to construct bounds on a causal treatment effect. With
continuous outcomes, meaningful bounds are more difficult to obtain because the
domain of the outcome is typically unrestricted. In this paper, we combine an
instrumental variable and subjective assumptions in the context of an obser-
vational cohort study of HIV-infected women to construct meaningful bounds on
the initial-stage causal effect of antiretroviral therapy on CD4 count. The
subjective assumptions are encoded in terms of the potential outcomes that are
identified by observed data as well as a sensitivity parameter that captures
the impact of unmeasured confounding. Measured confounding is adjusted using
the method of inverse probability weighting (IPW). With extra information from
an IV, we quantify both the causal treatment effect and the degree of the
unmea- sured confounding. We demonstrate our method by analyzing data from the
HIV Epidemiology Research Study.
