  With the rise of network science old topics in ecology and economics are
resurfacing. One such topic is structural stability (often referred to as
qualitative stability or sign stability). A system is deemed structurally
stable if the system remains stable for all possible parameter variations so
long as the parameters do not change sign. This type of stability analysis is
appealing when studying real systems as the underlying stability result only
requires the scientist or engineer to know the sign of the parameters in the
model and not the specific values. The necessary and sufficient conditions for
qualitative stability however are opaque. In order to shed light on those
conditions root locus analysis is employed. This technique allows us to
illustrate the necessary conditions for qualitative stability.
