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Chapter 13. Introduction to Survival Ana... > Survival Analysis Procedures - Pg. 254

254 ! Chapter 13: Introduction to Survival Analysis Procedures continue to participate) before termination of data collection. In either case, only a lower bound on the failure time of the censored observations is known. These observations are said to be right censored. Thus, an additional variable is incorporated into the analysis to indicate which failure times are observed event times and which are censored times. More generally, the failure time might only be known to be smaller than a given value (left censored) or known to be within a given interval (interval censored). There are numerous possible censoring schemes that arise in survival analysis. The monograph by Maddala (1983) discusses several related types of censoring situations, and the text by Kalbfleisch and Prentice (1980) also discusses several censoring schemes. Data with censored observations cannot be analyzed by ignoring the censored observations because, among other considerations, the longer-lived individuals are generally more likely to be right censored. The method of analysis must take the censoring into account and correctly use the censored observations as well as the uncensored observations. Another characteristic of survival data is that the response cannot be negative. This suggests that a transformation of the survival time such as a log transformation might be necessary or that spe- cialized methods might be more appropriate than those that assume a normal distribution for the error term. It is especially important to check any underlying assumptions as a part of the analysis because some of the models used are very sensitive to these assumptions.