Beginning with the planning stages of the National Assessment of Educational Progress (NAEP), careful attention has been given to the design of efficient probability sampling methods for the selection of class-age respondents and the assignment of test packages. With these methods, it is possible for NAEP researchers to make relatively precise statements about population characteristics on the basis of fairly small samples. The purpose of this monograph is to describe what is meant by relatively precise statements about population characteristics and to show how NAEP sample data are being used to gauge the accuracy of reported results. The levels of precision for Year 01 and 02 were compared, and the overall precision was improved in Year 02. The sampling error methodology developed for the Year 02 sample was applied to calculate Year 03 and 04 sampling errors. A major redesign of NAEP's primary sample was initiated for the Year 05 assessment. The Year 06 (1974-75) NAEP in-school primary sample was an independent replicate of the Year 05 sample selected from the deeply stratified primary unit frame developed for the 1973-74 survey. Four non-overlapping samples were to be used successively for Years 07 through 10. Primary type of information provided by report: Procedures (Sampling) (Evaluation). (BW)