Dear SPM’ers,
I have few questions regarding the use of covariates in the statistical analysis with SPM. It is not clear to me if covariates in SPM are treated the same as if using a stats software (e.g. SPSS).
I want to perform an Full factorial ANOVA with SPM on structural data (1 factor 3 levels) and I want to control for (“remove”) the effect of age (continuous variable), gender (categorical, 2 levels), handedness (categorical, 3 levels) and ethnicity (categorical, 4 levels).
1) Under what assumptions is it correct to use categorical nuisance variables (e.g. gender, handedness) given that using stats software as for example SPSS, covariates should be continuous and not categorical?
2) If variables as for example age, gender or handedness do not differ across groups it is still correct to remove their effects, assuming that being right-handed, left-handed or ambidextrous (or male and female, older or younger) could result in brain differences?
3) To justify the use of a covariate should I use statistical criteria or theoretical criteria that take into account the fact that we are working with the human brain?
Thank you very much and I do apologize if they sound very basic questions.
Regards,
Stefania
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