Hello,
I am currently starting to familiarize myself with different variations of the ACE-model as well as nuclear twin family designs in OpenMx and although coming from econometrics I dreaded working with SEMs so far I find the ride quite exhilarating.
While trying to reproduce certain results from the literature to broaden my understanding I sometimes encounter a strange behavior where I obtain a parameter estimate which is quite close to what I would expect with the only issue being that it is actually negative.
Once I set lower bounds for all parameters where negative values should not occur the problem vanishes and the estimate in question just switches to being almost the same but positive, therefore allowing me to get to the expected result.
As this happened multiple times with different variables and models by now I am confused about the significance of this behavior. Does it indicate that there exists a general underlying problem with my setup? Might I have missed some crucial component which ought to be specificed in every model? Or is this just something I should expect in twin-models and therefore always set lower bounds to eliminate the possibility of negative variance estimates?
Thanks for your help and continuing development of this great piece of software!
Tobias