JSM 2005 - Toronto

Abstract #303308

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Legend: = Applied Session, = Theme Session, = Presenter
Activity Number: 223
Type: Topic Contributed
Date/Time: Tuesday, August 9, 2005 : 8:30 AM to 10:20 AM
Sponsor: Biometrics Section
Abstract - #303308
Title: Dependence of Local Sensitivity to Nonignorability on the Assumed Dropout Mechanism
Author(s): Hui Xie*+ and Daniel Heitjan
Companies: Boston University and University of Pennsylvania
Address: 812 Memorial Drive, Cambridge, MA, 02139, United States
Keywords: link function ; missing data ; sensitivity analysis ; longitudinal data
Abstract:

When a longitudinal study is beset with dropout and there is concern the dropout mechanism is nonignorable, a sensitivity analysis is in order. Troxel et al. (2004) and Ma et al. (in press) proposed to accomplish this by computing an index of local sensitivity to nonignorability (ISNI)}, which measures the sensitivity of estimates to small departures from ignorability. One restriction of their approach is that one must specify a model for the missingness mechanism; applications so far have used either the logit, complementary log-log, or probit link (Troxel 2004; Ma and Heitjan 2004; Ma 2005). Because this assumption is typically of questionable validity, there is concern a sensitivity analysis that does not account for misspecification of the link function may be misleading. In this article, we relax the assumption of a known link by considering larger classes of models that include logistic regression as a special case. Through examples, we show how the extended ISNI can identify circumstances under which the ignorable analysis may be more or less sensitive to nonignorable dropout.

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