SEDL / STP / STP1007-EB / STP10299S

Effects of Withdrawals on Tests Involving Censored Data in Toxicology Experiments

Groggel, D
Associate professors and professor, Miami University, Oxford, OH

Schaefer, R
Associate professors and professor, Miami University, Oxford, OH

Skillings, J
Associate professors and professor, Miami University, Oxford, OH

Pages: 18    Published: Jan 1988

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Abstract

Censoring can occur with some response variables that are frequently of interest in toxicology studies (for example, time to maturation or lifetime). This censoring can be in the form of fixed right censoring or a withdrawal, and its presence can have a major impact on the analysis of the data. In this paper we consider the effect of censoring on statistical procedures that are commonly used in toxicology experiments. The tests examined in this study are the Gehan and log rank tests which are available on the major statistical packages (SAS, SPSSX, and BMDP). The application of these tests for k ⩾ 2 samples and for multiple comparisons is considered. The effect of withdrawals on the performance of these tests is found to be quite different than the effect of fixed right censored observations. Our study results indicate that these tests can lead to incorrect or misleading inferences when there is a high percentage of withdrawals. The tests may have very poor sensitivity to treatment differences, and the actual significance levels may be quite different from what the user specifies.