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Biostatistics Advance Access published online on October 8, 2009
Biostatistics, doi:10.1093/biostatistics/kxp034
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© The Author 2009. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org.

Semiparametric estimation of the average causal effect of treatment on an outcome measured after a postrandomization event, with missing outcome data

Peter B. Gilbert*

Department of Biostatistics, University of Washington, Seattle, WA 98105, USA and Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North, Seattle, WA 98109, USA

Yuying Jin

Department of Biostatistics, University of Washington, Seattle, WA 98105, USA

pgilbert{at}scharp.org

In the past decade, several principal stratification–based statistical methods have been developed for testing and estimation of a treatment effect on an outcome measured after a postrandomization event. Two examples are the evaluation of the effect of a cancer treatment on quality of life in subjects who remain alive and the evaluation of the effect of an HIV vaccine on viral load in subjects who acquire HIV infection. However, in general the developed methods have not addressed the issue of missing outcome data, and hence their validity relies on a missing completely at random (MCAR) assumption. Because in many applications the MCAR assumption is untenable, while a missing at random (MAR) assumption is defensible, we extend the semiparametric likelihood sensitivity analysis approach of Gilbert and others (2003) and Jemiai and Rotnitzky (2005) to allow the outcome to be MAR. We combine these methods with the robust likelihood–based method of Little and An (2004) for handling MAR data to provide semiparametric estimation of the average causal effect of treatment on the outcome. The new method, which does not require a monotonicity assumption, is evaluated in a simulation study and is applied to data from the first HIV vaccine efficacy trial.

Keywords: Causal inference; HIV vaccine trial; Missing at random; Posttreatment selection bias; Principal stratification; Sensitivity analysis

1 To whom correspondence should be addressed.

Received December 30, 2008; revised July 29, 2009; accepted for publication July 30, 2009.


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