Dear all
I have a twin dataset with proband ascertainment (i.e. at least one proband in each pair is affected). I cant seem to figure out how in incorporate this in my ACE model - can anyone help? I have found several examples involving ACE models in twin data (for example https://openmx.ssri.psu.edu/svn/tags/stable-1.2/demo/UnivariateTwinAnalysis_PathRaw.R) however none of these speak of how to take proband ascertainment into account.
I have read that i read to include ascertainment probabilities, i.e. 0 for the healthy-healthy pairs (the ones i dont have), 1 for the concordant pairs and a free parameter between 0 and 1 for the disconcordant pairs. Does anyone know where to add this in the code and how??
I have read the manual on openmx and it didnt give me an answer..
Best wishes
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Dear Lisa,
There isn't a simple way in OpenMx (as far as I know) to deal with ascertained samples especially when you go beyond a simple Univariate model. In general one would have to specify a user-defined fit function that corrects for the ascertainment. This could either be -2* the log likelihood of the data or a chi-sq test i.e. [sigma(O-E)2/E] , where O are the observed cell proportions and E the expected proportions which are worked out with the function omxMnor e.g.
mxAlgebra(expression=omxMnor(expCovDZ, expMean, cbind(-Inf,-Inf), cbind(T,T)), name="bothBelowDZ")
Best if you mail me your specific details to see if I can help you to write a script by adapting one of mine. You can contact me at:
fruhling.rijsdijk@kcl.ac.uk
Thank you very much for your reply, i have sent you an email with further details. I look forward to hearing from you, hope you can help!