At this point in the model fitting, we returned to examine the need for a shared environmental common factor. Perhaps the poor fit for model 7 reflected the need for more than one familial factor (that is, genetic or shared environmental) to explain the pattern of findings for the AD criteria rather than more specifically the need for a shared environmental common factor. Indeed, in the presence of multiple genetic factors, when we dropped the shared environmental common factor from model 16 to produce model 19, we saw less than a unit change in the –2 log likelihood and a substantial improvement in the AIC.
