We want to do a 1-factor 3-levels linear trend analysis. In the feat3 lecture and “Appendix_A:_Brief_Overview_of_GLM_Analysis” document, linear trend across levels is investigated for three levels (low medium high) using the t-contrast [-1 0 1]. Is that really a linear trend analysis? Is it not the case that when condition medium is set to zero then it does not contribute to the contrast, and that t-contrast [-1 0 1] therefore reflects high > low?
The contrasts must be orthogonal to each other, e.g. to not mix average activation with linear increase. Is it then only possible to do a linear trend analysis for an even number of levels?
Any feedback will be highly appreciated.
Best regards
Hallvard
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