The Healthcare Cost and Utilization Project (HCUP) National (Nationwide) Inpatient Sample NIS is based on data from community hospitals, which are defined as short-term, non-Federal, general, and other hospitals, excluding hospital units of other institutions (e.g., prisons). The NIS includes obstetrics and gynecology, otolaryngology, orthopedic, cancer, pediatric, public, and academic medical hospitals. Excluded are community hospitals that are also long-term care facilities such as rehabilitation, psychiatric, and alcoholism and chemical dependency hospitals. Beginning in 2012, long-term acute care hospitals (LTACs) are also excluded from the sampling frame. However, if a patient received long-term care, rehabilitation, or treatment for psychiatric or chemical dependency conditions in a community hospital, the discharge record for that stay will be included in the NIS.
The NIS is sampled from the HCUP State Inpatient Databases (SID). Beginning with the 2012 data year, the NIS is a 20 percent sample of discharges from all community hospitals participating in HCUP in that data year. For data years 1988 through 2011, the NIS was a 20 percent sample of community hospitals and included all discharges within sampled hospitals. The national estimates presented in this section of Fast Stats were developed using the NIS Trend Weight Files for consistent estimates across all data years (e.g., LTACs were removed from analysis using trend weights). National estimates for data years prior to 2012 use the NIS Trend Weights Files for consistent estimates across all data years. Information by community-level income is only reported from 2003 forward because of inconsistent definitions over time in the income-related data elements in the NIS. Costs are only reported from 2000 forward because HCUP Cost-to-Charge Ratios (CCRs) are unavailable prior to 2000.
The Data Characteristic in the dataset include Inpatient stay, Age, Sex, Expected Payer, Community level income and Hospitalization type. Inpatient Stays: The unit of analysis in the NIS is the hospital discharge (i.e., the inpatient stay), not a person or patient. This means that a person who is admitted to the hospital multiple times in one year will be counted each time as a separate “discharge” from the hospital. Counts are summarized by discharge year. There were no exclusions applied to the data (e.g., transfers to another acute care hospital are included as separate hospital stays).
Age: Age refers to the age of the patient at admission. Discharges missing age are excluded from results reported by age. Sex: All non-male, non-female responses are set to missing. Discharges with missing values for sex are excluded from results reported by sex. Expected Payer: The “expected payer” data element in HCUP databases provides information on the type of payer that the hospital expects to be the source of payment for the hospital bill. Information is reported by the following expected primary payers: Medicare, Medicaid, private insurance, and the uninsured. Uninsured discharges include records in which the expected primary payer was self-pay, charity, and no charge. Discharges for other types of payers (e.g., Worker’s compensation, Indian Health Service, State and local programs) are not reported. Discharges missing expected payer are excluded from results reported by expected payer. Community-Level Income: Community-level income is based on the median household income of the patient’s ZIP Code of residence, with quartiles defined using the U.S. population. Over time, the data element in the NIS for community-level income has changed definitions. Starting in data year 2003, the cut-offs for the quartile designation are determined annually using ZIP Code demographic data obtained from the Nielsen Company, a vendor that compiles and adds value to data from the U.S. Bureau of Census. Nielsen uses intercensal methods to estimate annual household and demographic statistics for geographic areas. The value ranges for the national income quartiles vary by year. Information by community-level income is only reported from 2003 forward because of inconsistent definitions over time in the income-related data elements in the NIS. Income quartile is missing if the patient is homeless or foreign. Discharges missing the income quartile are excluded from results reported by community-level income.
Hospitalization Type: “Coding criteria for the six hospitalization types are based on International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) diagnosis codes, Clinical Classifications Software (CCS) categories, and diagnosis-related groups (DRGs). There are approximately 14,000 ICD-9-CM diagnosis codes. The Clinical Classifications Software (CCS) categorizes ICD-9-CM diagnosis codes into a manageable number of clinically meaningful categories. This clinical grouper makes it easier to quickly understand patterns of diagnoses. DRGs group patients according to diagnosis, type of treatment (procedure), age, and other relevant criteria. Each hospital stay has one assigned DRG. Each discharge was assigned to a single hospitalization type hierarchically, based on the following order: maternal, neonatal, mental health, injury, surgical, and medical. All discharges are categorized in one of the six mutually exclusive types of service lines.