Regression Coefficients Estimated Incorrectly When Centered Predictors Used

May 23, 2007

to getting accurate regression coefficients from a dataset with *small* (standard) numbers, which contains centered predictors. More specifically, I have a dataset with 18 observed data points containing a criterion (y), a centered predictor variable (x), another centered predictor variable (z), and the interaction of the two centered predictor variables (xz). This multiple regression equation is structured to test for interactions between the two continuous predictor variables (x and z) as prescribed by Aiken and West (1991) in their classic book.

When I run the regression in Excel with the centered predictors, some of the regression coefficents in the output are estimated to be 0, although they are clealry *not* 0 as estimated by SPSS 14.0.2. I have spent many hours troubleshooting this problem (and searched many forums on the internet) and still do not know why this is happening.

Initially, I thought the problem might have to do with the cross-product of the centered predictors, but even just doing a regression with one of the centered predictors (for certain centered predictors) yields a regression coefficient of 0 (although it should be non-zero as per SPSS 14.0.2). When doing these multiple regressions with non-centered predictors, all regression coefficients are estimated accurately.

I was wondering if anyone had any insights on why I am experiencing these problems.... If anyone wants a sample of some test data I have used to troubleshoot these problems, you can download a file from: [url] and/or email me at (email deleted by Mod) for more datasets or questions.

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[COLOR=#333333][/COLOR]

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