Hi John,

Please see below:

On 13 February 2017 at 21:48, John anderson <[log in to unmask]> wrote:

Dear FSL experts,
I have two groups of subjects and I want to study the difference between the groups using the command :
randomise -i 4Dmerged.nii.gz -vw -m mask.nii.gz -d design.mat -t design.con -n 5000 -T

1. In order to study the difference between the groups without covriates. I used in my design the following:
1 0
1 0
1 0
.
.
.
0 1
0 1
0 1
.
.
.
etc

2. I want to include age covariate: I demeaned age between the groups and included it in a third column in the design.

3. I want to include age and gender covariates: I included the demeaned age in a third column of the design, and I have created a fourth column and gave (male=1, female=0).
Regarding this analysis, are there nay rules to choose (0, 1 ) for categorical variables (e.g. male/female). When I gave one for male, and zero for female, I found difference between the groups. If I choose Zero for male and one for female, the difference disappear. Why this happen? How this can be explained?

Provided that sex is used only as nuisance (i.e., it is not part of any contrast, nor involved in any interaction), then coding as 0 vs 1, 1 vs 0, +1 vs -1, -1 vs +1, pi vs. sqrt(2), etc, makes no difference on the contrasts being tested.

If, however, you test the effect of sex, then you need to know which was coded a higher or lower value, so you can tell the direction of the effect. In that case, the results will change vastly: it's as testing A>B vs. B>A.

All the best,

Anderson

I highly appreciate your valuable comments!

John