We define the robustness of a level set homology class of a function f : X R as the magnitude of a perturbation necessary to kill the class. Casting this notion into a group theoretic framework, we compute the robustness for each class, using a connection to extended persistent homology. The special case X = R3 has ramifications in medical imaging and scientific visualization. Keywords. Topological spaces, continuous functions, level sets, perturbations, homology, extended persistence, well groups, well diagrams, robustness.