Lenz et al2(p14) claimed that the TOUS is a middle-range theory that “was proposed as a means for integrating existing information about a variety of symptoms.” The purpose of the TOUS is to understand multiple symptoms that occur at the same time to explain the symptoms as well as to guide research and nursing practice.The TOUS has 3 major concepts: influencing factors, symptoms, and performance. Lenz et al described relationships among the 3 major concepts as well as associations between subconcepts of the
major concepts, which indicate narrower propositions between the concepts. The TOUS was developed specifically for application in nursing practice and research. Because the TOUS has relatively concrete concepts and propositions and a goal of understanding multiple unpleasant symptoms, the TOUS can be characterized as a middlerange explanatory theory. In addition, the theory can be used to develop preventive interventions to adjust influencing factors for multiple symptoms,and the effectiveness of use of the TOUS in intervention on patientoutcomeshasbeenshowninresearch.17 Therefore, the theory can also be viewed as a middle-range predictive theory.