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Bivariate twin model(ACEorADE)

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Fengxia's picture
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Joined: 11/02/2017 - 03:33
Bivariate twin model(ACEorADE)

Hello,

Recently, I have done a bivariate models.BUT I am confused with how to choose ACE or ADE in the bivariate models? I have tried using ICC in each variable apartly, BUT the fitting model one is ACE when another is ADE. NOW the question is that which one bivariate model I should use ? ACE-ACE or ADE-ADE or ACE-ADE? If the the answer is ACE-ADE, how to write the script?

Thanks for some help!

Cheers,
Fengxia

AdminRobK's picture
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Joined: 01/24/2014 - 12:15
don't split up sample

I'm guessing this is a follow-up to your thread about your triphenotype analysis of sleep duration, diabetes, and HbA1c, right? I really don't think it's wise to divide your sample into two overlapping subsamples and run separate analyses on each. You can incorporate the U-shaped trend into your triphenotype model by creating a dummy-coded covariate (a "definition variable") for sleep duration--make the 6-8h group the reference group, and create one indicator variable for being in the <6h group and another for being in the >8h group. Condition the liability-scale mean of diabetes on these dummy variables, and fix the threshold to zero to identify the model. If the regression coefficients for the two indicator variables are both positive and differ significantly from zero, then you have results consistent with your logistic regression analyses.

Fengxia's picture
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Joined: 11/02/2017 - 03:33
definition variable?

Thanks for your suggestion!

YES, but my analysis is biphenotype, that's sleep duration and diabetes, sleep duration and HbA1c separately. According to my understanding about "definition variable" for sleep duration, I should define sleep1 with 6-8h=0 and <6h=1, sleep2 with sleep1 with 6-8h=0 and >8h=1? BUT there will have missing data in sleep1 when sleep=>8h, and sleep2 when sleep=>8h, then I can't impute all my sample. How can I solve this problem?

If I have sleep1 and sleep2, the model would be sleep1-sleep2-diabetes ?

Finally, I really hope to receive your suggestion about ACE-ACE or ADE-ADE or ACE-ADE?

AdminRobK's picture
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Joined: 01/24/2014 - 12:15
dummy coding

Are you not familiar with dummy coding? You'd have two indicator variables: one for <6h ('sleep1', say), and one for >8h ('sleep2', say). So, 6-8h would be scored (0,0), <6h would be scored (1,0), and >8h would be scored (0,1). You would condition either the mean or the threshold of diabetes on the two variables via regression, in addition to how you're currently conditioning on age.

The thing is, though, now that I think about it, I don't think the resulting model would be identified. It would be sort of like regressing diabetes onto sleep duration while simultaneously estimating a covariance between the two phenotypes. Biometrically decomposing a U-shaped relationship between two ordinal phenotypes doesn't seem so easy to me now.

Finally, I really hope to receive your suggestion about ACE-ACE or ADE-ADE or ACE-ADE?

If all three of those models seem plausible to you, why not try all three, and compare their fit to the data? Try adapting a script for your purposes, and post again for help if you get stuck. Note that with the ACE-ADE model, there would be only one free path from C, going to the first phenotype, and only one free path from D, going to the second phenotype. Thus, only A and E would contribute to the phenotypic covariance, and only A would contribute to the cross-twin cross-trait covariance.

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