Identification of Causal Effects in the Presence of Nonignorable Missing Outcome Values

• Activity Number: 311
• Type: Invited
• Date/Time: Tuesday, July 31, 2012 : 10:30 AM to 12:20 PM
• Sponsor: Social Statistics Section
• Author(s): Fabrizia Mealli*+ and Alessandra Mattei and Barbara Pacini
• Companies: University of Florence and University of Florence and University of Pisa
• Keywords: Bounds ; Causal Inference ; Instrumental Variables ; Missing Outcomes ; Principal Stratification

Abstract

We consider a new approach to identify the causal effects of a binary treatment when the outcome is missing on a subset of units and dependence of nonresponse on the outcome cannot be ruled out even after conditioning on observed covariates: The missing mechanism is nonignorable. We provide sufficient conditions under which the availability of a binary instrument for nonresponse allows us to partially identify causal effects both in the whole population and in some latent subgroups of units, defined by their nonresponse behavior in all possible combinations of treatment and instrument, named Principal Strata. Examples are provided as possible scenarios where our assumptions may be plausible; they are used to discuss the key role of the instrument for nonresponse in identifying average causal effects in presence of nonignorable missing outcomes and provide some recommendations on study designs for causal inference.