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Hi Silvia - On 28 April 2011 11:40, Silvia Juanes <[log in to unmask]> wrote: > Dear all, > > > I have a question regarding Glm contrasts, > > We propose the following analysis: > > We have two groups and a continuous variable and we want to know whether > there is a greater correlation in a group compared to the other. > > > My EV's are: > > EV1--> the control group, > EV2--> the patient group, > EV3--> the variable for controls, > EV4--> the variable for patients. > > > The design matrix is: > 1 0 x 0 > 1 0 x 0 > . . . > > 0 1 0 x > 0 1 0 x > . . . > > > The contrasts that we want to test are: > > 0 0 1 -1 - (because we want to see if the correlation is more > significative in controls vs patient) > > > 1 -1 1 -1 - (I think this contrast means if the correlation is more > positive in controls compared to patients) > No, this second contrast doesn't seem right. The first contrast 0 0 1 -1 tells you where the regression coefficient is larger in controls, and flipping the sign - 0 0 -1 1 - will tell you where the reverse is true. Eugene > However, in the last contrast, in addition to the effect that we are > testing, there is another effect that is "confounded": whether the > correlation is more negative in the patient's group compared to the > control's group. This effect is not a problem for my hypothesis but what i > would like to know is whether the second contrast is correct in order to > test what we have explained. > > Many thanks to you all, > > Silvia > > > > >