Hi,

I was wondering what one should do if the estimates of the model have a negative component for the variances of a given component. I am familiar that in the direct variance approach paper it mentions there can be negative A estimates, and this can make biological sense. Similarly, at least for the ACE/ADE base model, there is a negative/positive parameter estimate relationship in which interpretability should win out (so whichever one is positive here, since the AIC will be the same in any case for ACE/ADE). However, if the A estimate is negative, or any of the submodels have a negative estimate, and this corresponds to the lowest AIC, is there a potential explanation to this, other than that the model is not well-suited (given the paper itself mentions there can be negative A estimates and biologically this may be feasible)?

I appreciate it!