Chapter 11. Analysis of Covariance

  • 11.1 Introduction 221

  • 11.2 Synopsis 222

  • 11.3 Examples 225

  • 11.4 Details & Notes 246

11.1 Introduction

Analysis of covariance (ANCOVA) provides a method for comparing response means among two or more groups adjusted for a quantitative concomitant variable, or 'covariate', thought to influence the response. The attention here is confined to cases in which the response, y, might be linearly related to the covariate, x. ANCOVA combines regression and ANOVA methods by fitting simple linear regression models within each group and comparing regressions among groups.

ANCOVA methods represent one of the most widely used statistical methods in clinical trials. Examples where ANCOVA might be applied include:

  • comparing cholesterol ...

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