I am finding that county is missing for about 2/3 of individual observations from the basic monthly files between 1997 and 2000. Is there documentation on the criteria used to determine which counties were reported and which ones hidden?
As noted on the COUNTY description tab: to preserve respondent confidentiality, not all counties are identified. Since not all counties have the necessary number of households required to preserve confidentiality, as determined by the Census Bureau, in recent years, only about 45 percent of households in recent years are located in a county that is identified. You can find links to lists of the identifiable counties for each year here.