Research results should not be taken at face value; some research is not well designed, and readers must be able to assess whether the research carried out actually supports the results or may be explained otherwise. Research reports are usually divided into introduction or literature review, methods, results, and discussion and conclusions. Basic components of a study include variables and constructs, dependent and independent variables, and hypotheses. The reader must determine whether the components are clear and relationships are identified. It is also necessary to understand the concepts of variance and control and their application to research design. Potential threats to the external validity of a study include the length of treatment and sample selection. Threats to internal validity may include the following: participant selection or grouping; participant mortality (withdrawal); effects that may occur simply in the passage of time; participant and researcher attitudes; validity (face, content, and construct) and reliability (stability and equivalence) of the measures used, and the effect of practice on results; and designs that lead to different treatment of groups or do not eliminate alternative explanations. It is also important to realize that not all threats to validity can be avoided, but can be acknowledged and controlled. (MSE)