 |
 |
Dear Mark, > I am trying to put a covariate into an SPM analysis. The covariate is > based on visual analogue scales, and I suspect that different subjects > have used different levels on the VAS to mean the same degree of the > variable I am trying to measure. I therefore wonder if I shouldn't be > using "specific fit" for this covariate. Is this a reasonable thing to > do? Very reasonable (and more comprehensive than simply mean correcting the subject-specific VAS scores before entering them as a single covariate). > Secondly when I put the covariate in as "specific fit" it then asks for > 11 items in each contrast vector. (I assume because I have 11 > subjects). I entered 2 contrasts, 1 1 1 1 1 1 1 1 1 1 1 and -1 -1 -1 > -1 -1 -1 -1 -1 -1 -1 -1, again is this reasonable? Yes it is. These contrasts will give you the average regression slopes over subjects. Note you can also test for differences among subjects (i.e. subject x VAS interactions) with the appropriate contrasts using this model. Bets wishes - Karl %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%