The report describes the fourth and final study in a program of research into the relationships between the characteristics of human tasks and the abilities required for task performance. The goal was to generate principles for identification of ability requirements from knowledge of the characteristics of a task and of variations in the conditions of task performance. Such knowledge has implications for selection and training of personnel. In this fourth study, possible interactions among task variations, ability profiles, and subject strategies were examined within the context of troubleshooting and problem-solving tasks previously studied. Subjects' strategies were defined in terms of their method of problem-solving under each level of task difficulty and perceptual complexity. Subgroups adopting one of several kinds of strategy were then analyzed to determine the relationships between abilities and strategies, and strategies and performance. The recurrent findings support the contention that abilities are sensitive to differences among tasks, making them more precise descriptors of performance than the broader rubrics (e.g., mental, motor; cognitive, noncognitive; etc.) which have been in vogue. In general, knowledge of a subject's problem-solving strategy was useful in obtaining a clearer understanding of ability requirements under different conditions of task performance. (Author)