Separate files are prepared for each annual survey.
Each file contains approximately 200,000 records that have a fixed-length format. Beginning in 1993, the layout of the public-use files was changed to be compatible with that of the multiyear files. This was done to allow for easily concatenating the annual files after 1993 with the multiyear files. A set of multiyear files with data for the 1979 to 1997 data years, combined and reformatted with standard coding of all variables, is also available. Unlike the single year files, the multiyear files separate records for newborn infants and non-newborns as distinct data files.