Dear Alfonso

Since we are part of a multi-center study, we have received a lot of resting state fMRI data (n>150) from different sites (n=9) that we plan to analyze. 
At the moment, we are having three covariates of no interest (age, gender, IQ) for our sample of patients and controls besides some covariates of interest (e.g., ADHD symptoms). We centered "age" and "IQ" values, and defined "gender" as "1=male", "2=female".


My question is: How do we define "site" as covariate of no interest in order to add "site" to our "2nd level covariates"?


Thanks a lot for your help!


Best regards
Julia