We drew on several data sources to estimate Medicare DSH payments: the Provider of Service (POS) file, the PPS Impact files for FY 1998 and FY2000, the Provider-specific File (PSF), and HCRIS files. Our starting point for a hospital listing was the CMS Provider of Service (POS) file. According to this file, there were 8,868 providers classified as "hospital" in the United States in FY1998. Of this total, we identified as our initial sample approximately 6,200 acute care hospitals that could be theoretically eligible to receive Medicare DSH payments. We used the information in the PPS Impact files and the PSF files to simulate DSH payments (see Appendix A for a more detailed explanation of the data sources). The PPS Impact files and the PSF file provided sufficient information to simulate Medicare DSH payments for 5,148 hospitals, or approximately 82% of hospitals in the initial sample. It appears that at least two-thirds of the hospitals with missing data could be hospitals that no longer exist or have merged with other hospitals. Although they are present in the 1998 POS file, they are not listed on other sources for 1998 or later, including the AHA survey.