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Understanding how effective high-level NICUs (neonatal intensive care unitsthat have the capacity for sustained mechanical assisted ventilation and highvolume) are compared to low-level NICUs is important and valuable for bothindividual mothers and for public policy decisions. The goal of this paper isto estimate the effect on mortality of premature babies being delivered in ahigh-level NICU vs. a low-level NICU through an observational study where thereare unmeasured confounders as well as nonignorable missing covariates. Weconsider the use of excess travel time as an instrumental variable (IV) tocontrol for unmeasured confounders. In order for an IV to be valid, we mustcondition on confounders of the IV---outcome relationship, for example, monthprenatal care started must be conditioned on for excess travel time to be avalid IV. However, sometimes month prenatal care started is missing, and themissingness may be nonignorable because it is related to the not fully measuredmother's/infant's risk of complications. We develop a method to estimate thecausal effect of a treatment using an IV when there are nonignorable missingcovariates as in our data, where we allow the missingness to depend on thefully observed outcome as well as the partially observed compliance class,which is a proxy for the unmeasured risk of complications. A simulation studyshows that under our nonignorable missingness assumption, the commonly usedestimation methods, complete-case analysis and multiple imputation by chainedequations assuming missingness at random, provide biased estimates, while ourmethod provides approximately unbiased estimates. We apply our method to theNICU study and find evidence that high-level NICUs significantly reduce deathsfor babies of small gestational age, whereas for almost mature babies like 37weeks, the level of NICUs makes little difference. A sensitivity analysis isconducted to assess the sensitivity of our conclusions to key assumptions aboutthe missing covariates. The method we develop in this paper may be useful formany observational studies facing similar issues of unmeasured confounders andnonignorable missing data as ours.

annals of applied statistics/the annals of applied statistics

Estimation of causal effects using instrumental variables with  nonignorable missing covariates: Application to effect of type of delivery  NICU on premature infants