Annu. Rev. Public Health 1997. 18:83–104 Copyright c  1997 by Annual Reviews Inc. All rights reserved CENSORING ISSUES IN SURVIVAL ANALYSIS Kwan-Moon Leung 1,2 , Robert M. Elashoff 1 , and Abdelmonem A. Afifi 1 1 Department of Biostatistics, School of Public Health, University of California, Los Angeles, California 90095: e-mail, moon@ucla.edu; 2 Quality Initiatives Division, Health Net, Woodland Hills, California 91367 KEY WORDS: survival analysis, right censoring, interval censoring, informative censoring, ignorability ABSTRACT A key characteristic that distinguishes survival analysis from other areas in statis- tics is that survival data are usually censored. Censoring occurs when incomplete information is available about the survival time of some individuals. We define censoring through some practical examples extracted from the literature in various fields of public health. With few exceptions, the censoring mechanisms in most observational studies are unknown and hence it is necessary to make assumptions about censoring when the common statistical methods are used to analyze cen- sored data. In addition, we present situations in which censoring mechanisms can be ignored. The effects of the censoring assumptions are demonstrated through actual studies. INTRODUCTION Survival analysis is used in various fields for analyzing data involving the duration between two events, or more generally the times of transition among several states or conditions. It is also known as lifetime data analysis, reliability analysis, time to event analysis, and event history analysis depending on the type of application. In this paper, the term survival time is used interchangeably with the terms risk period, lifetime, failure time, and time to a certain event. To determine the survival time, we need to define two time points: the time of origin, i.e. the time at which an original event, such as birth, occurs and the time of failure, i.e. the time at which the final event, such as death, occurs. A 83 0163-7525/97/0510-0083$08.00 Annu. Rev. Public Health 1997.18:83-104. Downloaded from www.annualreviews.org Access provided by 54.163.196.8 on 03/16/22. For personal use only.