Data Source

The national estimates presented in this section of Fast Stats are from the HCUP National (Nationwide) Inpatient Sample (NIS). The 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).

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 nonmale, nonfemale 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., Workers' compensation, Indian Health Service, State and local programs) are not reported. More information on expected payer coding in HCUP data is available in HCUP Methods Series Report #2014-03, "An Examination of Expected Payer Coding in the HCUP Databases" (multiple documents). 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.

Principal Diagnosis

The principal diagnosis is that condition established after study to be chiefly responsible for the patient's admission to the hospital. Diagnoses in the NIS are reported using the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM). 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.

Results are reported by the CCS code of the principal diagnosis and list the top 10 most common principal diagnoses for each data year. The top 10 ranking is based on the weighted number of stays.

Results can be displayed with maternal and neonatal stays included or excluded from the ranking. This option is provided because maternal and neonatal discharges account for nearly a fourth of all hospital discharges in a year and the majority are low complexity, low cost stays. Maternal and neonatal stays are defined using the principal diagnosis CCS 176 through 196 for maternal and CCS 218 through 224 for neonatal.

Rate of Stays per 100,000

The rate of stays includes the HCUP number of stays in the numerator and the U.S. resident population in the denominator (with a multiplier of 100,000). The denominator is consistently defined with the numerator (i.e., rates for females use HCUP counts and population counts specific to females). Population data are 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. Rates are not reported by expected payer because payer-specific population denominators are not consistently available for the study period.