JSM 2003 Online Program

Abstract #300138

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Activity Number:	332
Type:	Contributed
Date/Time:	Wednesday, August 6, 2003 : 8:30 AM to 10:20 AM
Sponsor:	Biopharmaceutical Section
Abstract - #300138
Title:	Some Unexamined Aspects of Analysis of Covariance in Pretest-Posttest Studies
Author(s):	Jitendra Ganju*+
Companies:	Chiron Corporation
Address:	4560 Horton St., Emeryville, CA, 94608-2916,
Keywords:	random baseline measurement ; conditional/unconditional analysis
Abstract:	The use of an analysis of covariance (ANCOVA) model in a pretest-posttest setting deserves to be studied separately from its use in other (non-pretest-posttest) settings. The following points will be made: (a) The ANCOVA model does not describe how data arrive. (b) For randomized studies it is impossible for nonparallel regression lines to exist. (c) Nonparallel regression lines may exist in observational studies even if the treatment effect is constant across all levels of the covariate. (d) Adjusting for the pretest variable in observational studies may actually introduce bias where none previously existed.

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