# FW: st: centering explanatory variables around zero

 From Cameron McIntosh To STATA LIST Subject FW: st: centering explanatory variables around zero Date Sun, 9 Aug 2009 22:49:28 -0400

```Yes, the interaction between a continuous variable and a nominal (or
ordinal) variable will just be a set of product terms (all k -1 dummies
x the continuous variable) that you will add to the model. These books will also help you make sense of your results:

Cohen, J., Cohen, P., West, S.G., & Aiken, L.S. (2003). Applied multiple regression/correlation analysis for the behavioral sciences. (3rd ed.).
Mahwah, NJ: Erlbaum.

Aiken, L.S., & West, S.G. (1991). Multiple regression: Testing and interpreting interactions. Newbury Park, London: Sage.

Cam
>
>> Date: Sun, 9 Aug 2009 14:31:26 -0500
>> To: statalist@hsphsun2.harvard.edu; statalist@hsphsun2.harvard.edu
>> From: Richard.A.Williams.5@ND.edu
>> Subject: Re: st: centering explanatory variables around zero
>>
>> 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:
>>
>> http://www.nd.edu/~rwilliam/stats2/l53.pdf
>>
>>
>> -------------------------------------------
>> 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
>> WWW: http://www.nd.edu/~rwilliam
>>
>> *
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>
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