Admission month (AMONTH) is derived from either the month of the admission date or the supplied admission month. A valid nonmissing month is assigned to AMONTH even if the admission year or day is invalid or missing. Therefore, it is possible to have a valid AMONTH when the admission date is invalid or missing.

If AMONTH is nonnumeric or out of range (month NE 1-12), then AMONTH is invalid (.A).

If the data source does not provide the admission month, then beginning in the 1998 data, AMONTH is not present on the HCUP files. In the 1988-1997 data, AMONTH is retained on the HCUP files and is set to unavailable from source (.B).