Thomas Snyder & Sally Dillow (PDF):

The data in this volume were obtained from many different sources–including students and teachers, state education agencies, local elementary and secondary schools, and colleges and universities–using surveys and compilations of administrative records. Users should be cautious when comparing data from different sources. Differences in aspects such as procedures, timing, question phrasing, and interviewer training can affect the comparability of results across data sources.
Most of the tables present data from surveys conducted by the National Center for Education Statistics (NCES) or conducted by other agencies and organizations with support from NCES. Some tables also include other data published by federal and state agencies, private research organizations, or professional organizations. Brief descriptions of the surveys and other data sources used in this volume can be found in Appendix A: Guide to Sources. For each NCES and non-NCES data source, the Guide to Sources also provides information on where to obtain further details about that source.
Data are obtained primarily from two types of surveys: universe surveys and sample surveys. In universe surveys, information is collected from every member of the population. For example, in a survey regarding certain expenditures of public elementary and secondary schools, data would be obtained from each school district in the United States. When data from an entire population are available, estimates of the total population or a subpopulation are made by simply summing the units in the population or subpopulation. As a result, there is no sampling error, and observed differences are reported as true.