Modeling "frailty": can a simple feedback model suffice?

This commentary offers several views of frailty, and provides an excerpt of the dynamic model proposed by Varadhan et al., noting that it is of a traditional, causal feedback type. Key features of the authors' model are examined, and some limitations identified. (Other more modern approaches are suggested, but not expanded.) In spite of its simplicity, the deterministic feedback model as presented does indicate that weakened connectivity among feedback loops is a possible basis of frailty. In contrast, a nodal-network style of modeling might emphasize nodal failure as well as (or instead of) connectivity failure as the dynamic signature of frailty.