I have twin data with a count variable that I want to fit with the traditional ACE model to estimate heritability of the phenotype, and model the count variable as Poisson. Therefore, due to the count variable, I specified this multiple groups model as a TYPE=MIXTURE with KNOWNCLASS = the two zygosity groups, mz and dz. After attempting to specify a residual variance or "specific environment" (E term in the ACE model), I learned that Poisson variables do not have the residual variance defined in Mplus, presumably because the mean and variance of a Poisson rv are equal.
Without the E term, it looks like the model is not identified. Examining the path model with tracing rules, it looks like the covariance for MZ twins is identical to the total variance of the phenotype, ie. both are 1*a^2 + 1*c^2, where a = the path coefficient for the additive genetic effect and c = path coefficient for common environment. And without the E term, Mplus gives me a "cov matrix not invertible" error, definitely a bad sign.
So, the Poisson ACE model does not exist and the Poisson AC model is not identified? And I should just "forgetahboutit"?
Bill H, MS, Dept Psychiatry, Wash U, St Louis
ps--analyzed as a continuous var with GROUP= specification as in Prescott (2004), the traditional ACE model runs fine