What do you want to do exactly? Test whether adding the age*group interaction is significant? If so, then you can just run model2 with a contrast looking at that term (or those terms). BTW, it would seem that the model they describe would be ill-conditioned because age*b2 and group*age*b3 would be redundant. doug

On 04/04/2013 11:04 AM, j janssen wrote:

Hi,

i read the following in a paper and would like to do the same:

"At each vertex in each hemisphere, the proportion of total variance in cortical thickness (CT) accounted for by 2 linear regression models was compared: One that did not include an age-by-group interaction term, Model 1: [CT = intercept + (group * b1) + (age * b2)] and one that did, Model 2: [CT = intercept + (group * b1) + (age * b2) + (group * age * b3)]. This resulted in an F ratio map for each hemisphere showing the degree to which the inclusion of an age-by-group term increased the proportion of CT variance that was accounted for."

i have set up the matrices for the 2 models. my questions:

1. do you think that i should test the effect of age in model 1

(contrast: 0 0 1) and the effect of age * group in model 2 (contrast 0 0 0 1 -1)? if not, what do you suggest?

2. do you have any idea how to combine these two analyses in a

resulting F ratio map?

thanks, -joost

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