Aporetic Conclusions When Testing the Validity of an Instrumental Variable

• Activity Number: 436
• Type: Invited
• Date/Time: Wednesday, August 6, 2014 : 8:30 AM to 10:20 AM
• Sponsor: Health Policy Statistics Section
• Author(s): Dylan Small*+ and Fan Yang and Jose Zubizarreta and Scott Lorch and Paul R. Rosenbaum
• Companies: University of Pennsylvania and Wharton School and Columbia University and Children's Hospital of Philadelphia and Wharton School
• Keywords: instrumental variable ; causal inference ; comparative effectiveness

Abstract

An instrument or instrumental variable is often used in an effort to avoid selection bias in inference about the effects of treatments when treatment choice is based on thoughtful deliberation. An instrument is a haphazard nudge to accept one treatment or another, where the nudge can affect outcomes only to the extent that it alters the treatment received. There are two key assumptions here: (i) the nudge is haphazard or essentially random once adjustments have been made for observed covariates, (ii) the nudge affects outcomes only by altering the treatment, the so-called exclusion restriction. These assumptions are often said to be untestable; however, that is untrue if testable means checking the compatibility of assumptions with other things we think we know. A test of this sort may result in an aporia, that is, a collection of claims that are individually plausible but mutually inconsistent, without clear indication as to which claim is culpable for the inconsistency. We discuss this subject in the context of our on-going study of the effects of delivery by cesarean section on the survival of extremely premature infants of 23-24 weeks gestational age.