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 LISTSERV Archives SPM Home SPM April 2008

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Re: Questions about getting learning effect

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Date:

Tue, 29 Apr 2008 18:11:54 -0700

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 ```Not quite. For the model with the pm the beta of the task is mean(10+[1:8]) and the beta of the pm is 1. The difference between the two is that the model without the pm has a much larger error term because you are mot modeling the linear trend in the data. For your example, try this: y = 10+[1:8]'; x1 = ones(8,1); % model without pm = only average effect x2 = [ones(8,1) [1:8]'-repmat(mean(1:8),8,1)]; % model with pm b1 = x1\y = 14.5 b2 = x2\y = [14.5 1] ssq1 = (y-x1*b1)' * (y-x1*b1) = 42 ssq2 = (y-x2*b2)' * (y-x2*b2) = 2.2404e-28 ~ 0 (perfect fit) Cheers, Jan Wu Xiang wrote: > Thanks Jan. Please further help check the below, I am not sure whether I > exactly understand how adding parametric modulator (pm) change the beta of > the task: > > given values of the 8 blocks of a task are 10+[1:8] > for the model without pm, the beta of the task would be mean(10+[1:8]) > for the model with pm, the beta of the task would be 10, and the beta of the > pm would be like 1(the slope is 1) > Is the above correct? > > Xiang > > -- Jan Gläscher, Ph.D. Div. Humanities & Social Sciences +1 (626) 395-3898 (office) Caltech, Broad Center, M/C 114-96 +1 (626) 395-2000 (fax) 1200 California Blvd [log in to unmask] Pasadena, CA 91125 ```

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