Checking factor level order

[Adam Teed]

Hello Aaron,

I have a 3 level group factor and I wanted to make sure of the direction of my contrasts being that changing the order of a posthoc contrast doesn't flip the sign. Are factor levels entered alphabetically or by first appearance? If it's the latter, that would mean it assigns levels as it enters contrasts images following the subject index, correct? Thus, can I just check the order of appearance by entering dat.FACTORNAME and read the descending order?

Thank you,

Adam

[Aaron Schultz]

Please see this post:
https://groups.google.com/forum/#!searchin/fmri_matlab_tools/Contrast$20direction%7Csort:date/fmri_matlab_tools/iocDrTBH9jc/bep9hmHaAAAJ

If the factor is 3 level, then the output stats should be F-stats
which will always be positive.

Off the top of my head, I believe the order for the design matrix is
based off of unique(factor,'stable') (though it may be unique(factor)
which will sort the levels alpha-numerically). For a three level
factor the design matrix will be formed as 1-2 and 2-3.

As covered in the thread listed above, the best thing to do is extract
a VOI and plot the data to ensure that you are interpreting
directionality correctly.

-Aaron

> --
> You received this message because you are subscribed to the Google Groups "Aaron's fMRI matlab tools." group.
> To unsubscribe from this group and stop receiving emails from it, send an email to fmri_matlab_to...@googlegroups.com.
> To post to this group, send email to fmri_mat...@googlegroups.com.
> To view this discussion on the web visit https://groups.google.com/d/msgid/fmri_matlab_tools/26612df5-d23e-4995-8ac6-daf162302d46%40googlegroups.com.
> For more options, visit https://groups.google.com/d/optout.