Hi everyone,
I want to test a bivariate model of two phenotypes. As I am very new to all this (and unfortunately not from the US and so can't attend the workshops) I researched the internet and ended up succeeding in running a bivariate model script and learning a bit about it (the script I took is from http://ibg.colorado.edu/cdrom2016/maes/UnivariateAnalysis/two/twoACEc.R).
However, I thought it would be wise to make sure there are no various sex-differences in my model. After reading a few threads in the forum I understood there are substantial problems in running a multivariate sex-limitation model. So several question:
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If the sex differences are not my focus but rather I just want to make sure it doesn't need to be included in the model, is it possible to run 2 separate univariate sex-limitation models? When doing that, I found in one trait the no sex-limitation model is significantly better than the sex-limitation non-scalar models, and in the other trait I found that there are no significant differences between the different sex-limitation models, but than the no sex-limitation is the most parsimonious and can be chosen. Is it ok to do that and than run my regular bivariate model with no sex-limitation? If not, what are other options I have?
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In the univariate sex-limitation syntax I found (https://ibg.colorado.edu/cdrom2018/deleeuw/sexLimitation/) there is no model for the scalar sex-limitation model. This was also the case for a different syntax I found. Is there a reason this is not tested? If it needs to be tested, do you have recommendation for where to find this syntax (I used the Boulder 2018 workshop one).
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If the answer to the first question is yes, and there seem to be no sex differences between sexes, I think I can than insert DZ other-sex twins into my no sex-limitation bivariate ACE model and increase the power of my model, is it correct? If so, what is the correct way to include DZ-O twins in the model?
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Last, as I mentioned, I found the bivariate syntax online. From the presentation attached to the syntax, I think the syntax is for a bivariate Cholesky model. How can I make sure from the script this is actually the case and it is not a different model (e.g., common factors)?
Thank you so very much!