Please see responses below.
On Tue, Jun 11, 2013 at 7:48 AM, Nicole Van Hoeck
<[log in to unmask]> wrote:
Dear SPM-users,
My design looks like this:
within subject design
1 group
1 factor with 3 levels (1A, 1B, 1C)
3 covariates: for each covariate I have one score per subject (testscores on executive funtion assessments)
3 questions:
1) When you use a one way
within subject Anova in SPM is the spericity assumption always
violated? (the bar underneath the design matrix is grey)
>>> The gray matrix below the design matrix represents the dependency of the data. If a box is grey, then the estimated beta for this column is dependent on the other betas with grey boxes. Sphericity is a difference issue that may or may not be violated in your case. Sphericity means that the covariances between different levels are all equal.
2) How do I create on 1st level a conjunction image of 1A&1B? (related to questions 3)
>>> You need to enter con_ images in the group map, so a conjunction is not possible. You could try to combine the betas by addition though.
3) I'm not sure how to incorporate the covariates.
-
I would like to know how the score on the executive functions
assessment correlate with the activation patters of the following
contrasts: 1A-1C, 1B-1C, and the conjunction of 1A&1B
- for example: one sample t-test (one covariate) with contrast "0
1" gives different results than multiple regression (3 covariates) with
contrast "0 1 0 0"
>>> Covariates will not change the relationship between the 3 levels; as such, I would recommend not including any covariates. Given your specific questions. I would form the contrasts of of your comparisons at the first-level, then use a one-sample t-test with a single covariate. Then assess with the covariate is significant.
Thank you so much for your help,