good question. I don't think there is anything wrong with multiplying two zs, 
but it might be more appropriate to multiply the raw covariates and then 
compute a z from that.

On 11/30/2020 4:07 AM, Marina Fernández wrote:

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Dear all,

I am doing a multiple regression analysis with 3 covariates of interest. I know 
that for testing two-way and three-way interactions I must create new variables 
by multiplying the covariates of interest. As Freesurfer transforms continuous 
variables to z score before applying statistic, is it correct to transform to z 
score the result of the previous multiplication? I am asking this because the 
result of multiplying two z transformed variables would be different.

Thanks a lot in advance.

Marina



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