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).

Central Distributor only - because of confidentiality restrictions, admission month (AMONTH) is suppressed on some records.

Special consideration was needed to handle the Massachusetts data beginning in the 2006 NIS. Fourth quarter data from sampled hospitals in Massachusetts were unavailable for inclusion in the 2006 and 2007 NIS. To account for the missing quarter of data, we sampled one fourth of the Massachusetts NIS discharges from the first three quarters and modified the records to represent the fourth quarter. To ensure a representative sample, we sorted the Massachusetts NIS discharges by hospital, discharge quarter, Clinical Classifications Software (CCS) diagnosis group for the principal diagnosis, gender, age, and a random number before selecting every fourth record. The following describes the adjustments made to the selected Massachusetts NIS records:

1. We relabeled the discharge quarter (DQTR) to four and saved the original discharge quarter in a new data element (DQTR_X).
2. We adjusted the admission month (AMONTH) by the number of months corresponding to the change in the discharge quarter.
3. We adjusted the total charges (TOTCHG and TOTCHG_X) using quarter-specific adjustment factors calculated as the mean total charges in the fourth quarter for all Northeastern NIS states (excluding Massachusetts) divided by the mean total charges in the first, second, or third quarter for all Northeastern NIS states (excluding Massachusetts).