[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: st: centering explanatory variables around zero

From   Richard Williams <>
To   "" <>, "" <>
Subject   Re: st: centering explanatory variables around zero
Date   Sun, 09 Aug 2009 14:31:26 -0500

At 12:50 PM 8/9/2009, Nikolaos Pandis wrote:
I have the following model

y=a + b1x1 + b2x2

y and x2 are continuous
x1=categorical with 3 levels

You shouldn't include x1 as is. You should break it up into dummy variables. e.g. if for x1, 1 = Catholic, 2 = Protestant, 3 = Others, it makes no sense to treat it as continous.

I find a large difference in the p-values for the explanatory variables between the interaction and no-interaction model.

I centered for the continuous variable x2 ((sum x2, gen cx2=x2-r(mean)).

I get similar p values for interaction and no-interaction models.

Question: How about the categorical explanatory variable? Is it appropriate to center this variable also? Does it make any sense?

No. Again, it shouldn't be in there in the first place; and if you did dichotomize it you still wouldn't center.

This handout goes over the advantages of centering and how to interpret results:

Richard Williams, Notre Dame Dept of Sociology
OFFICE: (574)631-6668, (574)631-6463
HOME:   (574)289-5227
EMAIL:  Richard.A.Williams.5@ND.Edu

*   For searches and help try:

© Copyright 1996–2017 StataCorp LLC   |   Terms of use   |   Privacy   |   Contact us   |   What's new   |   Site index