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Trend Weights for HCUP NIS Data
The NIS is a set of longitudinal hospital inpatient databases included in the HCUP family. These databases are created by AHRQ through a Federal-State-Industry partnership.
Brief Summary

For 1993-2011, use the hospital-level NIS Trend Weights Files, which are available for downloading below. There is an ASCII file and a SAS load program for each year. The trend weight files should be merged onto the original NIS files by Year and HOSPID. For years prior to 2012, the trend weight (TRENDWT) should be used in place of the original discharge weight (DISCWT) to create national estimates for trends analysis that are consistent with 2012 data onward. For 2012 and after, no trend weight is needed, and the regular discharge weight (DISCWT) should be used.

Trend Weights for 1993-2011 that are Consistent with the Redesigned NIS (Beginning in 2012)

The NIS is available yearly, beginning with 1988, allowing analysis of trends over time. Analyses of time trends using the HCUP NIS are recommended from 1993 forward.

The HCUP National Inpatient Sample (NIS) was redesigned beginning with 2012 data to improve national estimates. To highlight the design change, beginning with 2012 data, AHRQ renamed the NIS from the "Nationwide Inpatient Sample" to the "National Inpatient Sample." The redesign incorporates three major types of changes:

  • Revisions to the sample design—the NIS is now a sample of discharge records from all HCUP-participating hospitals, rather than a sample of hospitals from which all discharges were retained.
  • Revisions to how hospitals are defined—the NIS now uses the definitions of hospitals and discharges supplied by the statewide data organizations that contribute to HCUP, rather than the definitions used by the AHA Annual Survey.
  • Revisions to enhance confidentiality—the NIS now eliminates State and hospital identifiers and other data elements that are not uniformly available across States.
The new sampling strategy is expected to result in more precise estimates than the previous NIS design by reducing sampling error. For many estimates, confidence intervals under the new design are about half the length of confidence intervals under the previous design. For a detailed description of the NIS redesign, please see the full NIS Redesign Report, available on the AHRQ-sponsored HCUP-US Website.

As a result of the changes implemented in the redesign, users should expect one-time disruptions to historical trends for counts, rates, and means estimated from the NIS, beginning with data year 2012. For 2012 we expect overall trends in discharge counts to decline by about 4.3 percent, overall trends in average length-of-stay to decline by about 1.5 percent, overall trends in total charges to decline by about 0.5 percent, and overall trends in hospital mortality to decline by about 2.0 percent. For a detailed description of the effects of the design changes on sample estimates, please see the NIS Redesign Report available on the HCUP User Support (HCUP-US) Website.

In order to facilitate analysis of trends using multiple years of NIS data, AHRQ developed new discharge trend weights for the 1993-2011 NIS. These weights were calculated in the same way as the weights for the redesigned 2012 NIS, and are designed to be used instead of the original NIS discharge weights for trends analysis. For trends analysis spanning 2012 and earlier NIS data, trend weights should be used prior to 2012 data to make estimates comparable to the new 2012 NIS design. The trend weights were developed for trends analysis, but they can be used for all analyses, as is done on the HCUPnet Web site. The following table enumerates the differences in weighted discharges:

YEAR Number of Discharges in the NIS, Weighted Number of Discharges in the NIS, Weighted with Trend Weight Difference HCUPnet Weighted Discharges Difference Comment
1993 34,715,985 33,736,753 -979,232 33,735,002 1,751 HCUPnet removed outlier records.
1994 34,622,203 33,149,768 -1,472,435 33,148,180 1,588 HCUPnet removed outlier records.
1995 34,791,998 33,647,121 -1,144,877 33,644,908 2,213 HCUPnet removed outlier records.
1996 34,874,386 33,386,097 -1,488,289 33,384,374 1,723 HCUPnet removed outlier records.
1997 35,408,207 33,232,257 -2,175,950 33,230,554 1,703 HCUPnet removed outlier records.
1998 34,874,001 33,923,632 -950,369 33,923,632 0  
1999 35,467,673 34,440,994 -1,026,679 34,440,994 0  
2000 36,417,565 35,300,425 -1,117,140 35,300,425 0  
2001 37,187,641 36,093,550 -1,094,091 36,093,550 0  
2002 37,804,021 36,523,831 -1,280,190 36,523,831 0  
2003 38,220,659 37,074,605 -1,146,054 37,074,605 0  
2004 38,661,786 37,496,978 -1,164,808 37,496,978 0  
2005 39,163,834 37,843,039 -1,320,795 37,843,039 0  
2006 39,450,216 38,076,556 -1,373,660 38,076,556 0  
2007 39,541,948 38,155,908 -1,386,040 38,155,908 0  
2008 39,885,120 38,210,889 -1,674,231 38,210,889 0  
2009 39,434,956 37,734,584 -1,700,372 37,734,584 0  
2010 39,008,298 37,352,013 -1,656,285 37,352,013 0  
2011 38,590,733 36,962,415 -1,628,318 36,962,415 0  
2012 36,484,846     36,484,846 0  

The new trend weights replace the earlier NIS Trend Weights and NIS Trends Supplemental Files that were developed for the 1988-1997 NIS following the 1998 NIS redesign. The report Using the HCUP Nationwide Inpatient Sample to Estimate Trends1, available on the HCUP-US Website under Methods Series, includes recommendations for trends analysis.

1 As of June, 2014, this report had not yet been updated for the new 2012 NIS design. However, the methods described in the report are still valid.

The trend weights are available for download as ASCII files along with SAS, Stata, and SPSS load programs. Each file contains the following three data elements:

Name Type Label
YEAR Numeric Calendar year
HOSPID Numeric HCUP hospital identification number
TRENDWT* Numeric Weight to discharges in the universe

All discharges from a given hospital (HOSPID) have the same discharge weight in a given year of the NIS. The trend weight files should be merged onto the original NIS files by Year and HOSPID. The trend weight (TRENDWT) should be used in place of the original discharge weight (DISCWT) to create national estimates for trends analysis. While TRENDWT was developed for trends analysis, it can be used for all analyses, as is done on the HCUPnet Web site. By design TRENDWT has a value of zero for some hospitals, and this does not impair the accuracy of the estimates.

*The 2000 file contains a second trend weight: TRENDWTcharge. For 2000, use TRENDWT to create national estimates for all analyses except those that involve total charges; and use TRENDWTcharge to create national estimates of total charges. For all other years, use TRENDWT for all estimates.

NIS Trend Weights files for download:

Internet Citation: HCUP NIS Trend Weights. Healthcare Cost and Utilization Project (HCUP). May 2015. Agency for Healthcare Research and Quality, Rockville, MD.
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Last modified 5/12/15