This thesis tests the validity of three theories purporting to explain the attrition of stop outs -- why students interrupt their studies with the intent to return to school. Data were gathered at two schools, Portland State University and Reed College. Two theories, those of social class and involvement, were tested at each school as contending explanations of attrition by path analysis of parsimonious models derived by factor analysis. These theories were found to explain little of the variance of attendance pattern (less than 4.3 percent) at either school. No particular lines of causation could be demonstrated at either school. The third theory, that of career planning, asserts that students interrupt their studies to re-evaluate their course of action upon recognizing that chances of employment in their field are not good. This proposition was supported by the data at Portland State University.