In my study I am trying to find whether factors like diabetes, hypertension, etc have a significant effect on the association between genotypes of candidate genes and the disease phenotype? Can anybody explain briefly how to perform the same using SPSS software?
I agree with Adrian that you need to look at the test for the interaction term. However, I believe it is customary to include all lower order terms for variables involved in an interaction. So I would include a first-order term for diabetes in the second model, like this:
p = b0 + b1g + b2d + b3gd + error
In this model, the test on b3 would be the test for effect modification.
p.s. - Adrian, how did you make subscripts in here?
The usual formal test of effect modification is by test of the interaction term. In other words, if p represents phenotype, g gentotype and d presence of diabetes, then I assume your base model looks like:
p = b0 + b1g + error, and you would add the term
p = b0 + b1g + b2dg + error
if b2 is statistically significant, then effect modification is present
I agree with Adrian that you need to look at the test for the interaction term. However, I believe it is customary to include all lower order terms for variables involved in an interaction. So I would include a first-order term for diabetes in the second model, like this:
p = b0 + b1g + b2d + b3gd + error
In this model, the test on b3 would be the test for effect modification.
p.s. - Adrian, how did you make subscripts in here?
Hi Ferdinamarie,
I agree with Bruce.
As you are asking how to do it on SPSS, it might be helpful to calculate (transform/compute variable) an interaction variable by simply multiplying g*d. Once you have done this you can incorporate the new variable into the model as Adrian and Bruce have shown.
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