Researchers often conduct pilot studies. These are designed to determine the feasibility of methods for studying specific research problems. After the research techniques are refined in a pilot study with a sample of convenience, a more definitive study with a more appropriate sample for generalizing might be conducted. Some researchers focus on developing and testing theories. A theory is a proposition or set of propositions that provides a cohesive explanation of the underlying dynamics of certain aspects of behavior. For instance, if the theory is correct, a researcher might predict that people with poor self-concepts will seek out negative social reinforcement while avoiding or rejecting positive reinforcement. Some researchers prefer to study purposive samples rather than random samples. When researchers write research reports on population studies, they should describe their populations in some detail. Traditionally, qualitative researchers use smaller samples than quantitative researchers.