Dear Freesurfer experts,
I would like to study the relationship between cortical thickness and one clinical variable with qdec, but correcting for age and gender.
Given that I have only one group and 2 covariates (one continuous, one dichotomic) I don't know how I should set the design of my analysis in qdec, especially for the gender variable.
I have tried two different ways (both DOSS design):
1) selecting Discrete = gender; Continuous = clinical measure; Nuisance factor = age and looking at the contrast called : "Does the correlation between thickness and clinical measure accounting for gender differ from 0? nuisance factor : age"
2) selecting : continuous = clinical measure; Nuisance Factor = age, gender (coded as 1 and 2) and looking at the contrast called : "Does the correlation between thickness and clinical measure differ from 0, nuisance factor : age, gender"
But the two contrasts give very different results, which I find very surprising. I exported cortical thickness at the peak significance of the clusters and tried to run a GLM myself in SPSS and it seems that coding gender as a continuous variable with two values (1 and 2) provides the most realistic results. However, I saw many times on the mailing list that you recommend to use gender as a discrete variable, so I am very confused. Could you explain me the difference between these contrasts and help me to identify which one will accurately identify the correlation between cortical thickness and my clinical variable correcting for the effect of age and gender.
Thank you in advance for your answer, Best regards Elisa