[R] multiple moderated regression steps

Frank Harrell f.harrell at vanderbilt.edu
Fri Jul 1 18:25:01 CEST 2011

That concern has nothing to do with centering variables before including them
in a model.

Your multiple significance testing strategy is not based on statistical
principles and will distort all inferences you obtain from the "final"

stamkiral wrote:
> variables were centered (except DV), because in social sciences zero is
> rarely a meaningful point on a scale (Cohen, Cohen, West, Aiken, 2006).
> for example in percieved social support questionnaire there is no value as
> zero. It is a Likert Type questionnaire and on questionnaire 1= strongly
> yes.....7= strongly no. So Aiken and West (1991), suggested to centered
> predictors and moderators to appoint zero a meaningful value to count in
> regression equation. also multilevel steps were run to avoid
> multicollinearity. according to results .05 were accepted as significant
> (in the coefficient table) and slope test were done for interactions. 
> for plottin slopes unstandardised regression coefficient were used.

Frank Harrell
Department of Biostatistics, Vanderbilt University
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