Dear Andersson and other FSL experts,
I was reading this post and had some questions regarding the model that is specified:
EV1: intercept
EV2: age
EV3: age^2
EV4: sex
To test the linear effect of age (positive or negative), use:
C1: [0 1 0 0]
C2: [0 -1 0 0]
To test the quadratic effect, create a third contrast that is:
C3: [0 0 1 0]
And define an F-test that combines C1 and C3. This F-test will be the test for quadratic effects.
a) Why is it important to model the intercept and what value should be assigned to this intercept EV?
b) Why is only 1 contrast explicitly specified for the quadratic effect (C3: 0 0 1 0)? Shouldn't the following also be modelled C4: 0 0 -1 0?
c) Why is it necessary to set up an F-contrast that combines C1 and C3 in order to test for quadratic effects? Wouldn't C3 and C4 probe this already?
Many thanks for the assistance!
Mauro
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