Treatment Effect Bounds: An Application to Swan-Ganz Catheterization

Abstract

We reanalyze data from the observational study by Connors et al. (1996) on the impact of Swan-Ganz catheterization on mortality outcomes. The Connors et al. (1996) study assumes that there are no unobserved differences between patients who are catheterized and patients who are not catheterized and finds that catheterization increases patient mortality. We instead allow for such differences between patients by implementing both the bounds of Manski (1990), which only exploits an instrumental variable, and the bounds of Shaikh and Vytlacil (2004), which exploit mild nonparametric, structural assumptions in addition to an instrumental variable. We propose and justify the use of indicators of weekday admission as an instrument for catheterization in this context. We find that in our application, the Manski (1990) bounds do not indicate whether catheterization increases or decreases mortality, whereas the Shaikh and Vytlacil (2004) bounds reveal that catheterization increases mortality at 30 days and beyond. We also extend the analysis of Shaikh and Vytlacil (2004) to exploit a further nonparametric, structural assumption -- that doctors catheterize individuals with systematically worse latent health -- and find that this assumption further narrows these bounds and strengthens our conclusions.