Hello all,
I sent this message some time ago but didn't have an answer.
I would really be interested to know what is the "mean" column created
in multiple regression. Can I use the contrast [1 -1] to compare the
effect of my regressor against the mean or I can use only [1 0] and
never put the mean in a contrast. My effects are coming highly
significant if I compare against the mean, but not if I compare the
regressor alone.
You can see the covariate matrix on my previous message:
https://www.jiscmail.ac.uk/cgi-bin/wa.exe?A2=ind0812&L=SPM&P=R35067
Thank you
Dorian
2008/12/10 Dorian P <[log in to unmask]>:
> Dear all,
>
> Hope the analysis are flowing for you :)
>
> We are running a multiple regression analysis in 2nd level to catch
> the differences of 3 levels of memory (20%, 40% and 60%). After
> inserting the con_*.img files they are weighted in a Covariate with
> 20, 40 or 60 (the vector corresponds to the scans in input).
>
> Now we have a design matrix of 2 columns, one is the covariate and the
> other the mean. We are running various contrasts ([1 -1], [-1 1]) etc,
> but we don't understand what are we contrasting exactly (even though
> the results are nice for what we expect).
>
> Can somebody help on the topic? Attached you can find the Covariates
> matrix and the example of the fitted response for one contrast.
>
> Best Regards.
>
> Dorian.
>
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