Few systems with which researchers work could truly be called linear. If a system is linear, outputs of that system will be in proportion to inputs. Second, if a system is linear, two trajectories at nearby points in phase space would evolve in close proximity. Whether studying physiologic responses of individuals or health problems of entire communities, researchers are faced with understanding apparently nonlinear systems in which very small inputs can result in disproportionately large outputs. In addition, research challenges scientists to describe and explain phenomena that seem to evolve very differently in apparently similar systems. In other words, we ask, "Why would a health care intervention for two essentially equivalent clients result in such different outcomes?" or "Why is a particular type of service effective in one community and ineffective in another when the two seem to be demographically indistinguishable?"