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Dear list, I've run into trouble with a pretty basic fMRI paradigm. We measured 10 patients suffering from a neuropsychiatric disorder and 10 controls. Both groups performed three types of tasks plus a rest condition (A-Rest-B-Rest-C-Rest...3x). When contrasting the tasks on the second level, there are meaningful differences across groups. So far, so good. Now, however, we also acquired behavioral measurements according to the speed subjects performed the tasks with (difficulty C > B > A). I am interested in activations correlating with the speed measurement, therefore I imagined a parametric design would be in order. Here's what I did: first I classified each task condition as being "fast", "medium" or "slow" as compared to the according group performance quantiles. Then I designed EVs for each of the three categories and entered them into the first level design matrix. Unfortunately, some subjects only exhibited "fast" speeds, leaving "slow" and "medium" regressors empty, thus resulting in a rank deficient design matrix. Even omitting the "medium" EV did not help, obviously. Others had two different speeds, no one had three types. How can I specify a design matrix of this type, preserving all three EVs for a higher level analysis? Or should I assign "slow", "medium" and "fast" to subjects on the second level? Thanks for your help - any comment is appreciated! Cheers Cornelius