The National Assessment of Educational Progress; Second Science Assessment No-Show Study assessed the magnitude and causation of nonresponse biases. A No-Show is defined as an individual who was selected as a sample respondent but failed to be present for regular assessment of the 17-year-old group. The procedure whereby a sample of eligible 17-year-old No-Shows were selected to take four specific No-Show assessment packages is briefly described. Estimates of biases due to nonresponse were made for the following domain variables: region; sex; race; size and type of community; derived parental education. These domains are outlined. Also documented are the domain estimation methodologies, terminology and the methods utilized for the computation of formulas. No-Show domain analysis results are briefly summarized in tables, but are detailed more fully elsewhere in separate appendixes. Results show that reliability estimates were positive and several were significant, indicating that regular assessment students generally performed better on assessment packages than did the no-shows. Separate exercise analyses of bias are considered in section 6 as an extension of the analyses covered by the study. Primary type of information provided by report: Procedures (Evaluation) (Sampling). (Author/AEF)