There are important differences between these two databases and depending on the research hypothesis, one database may be preferable to the other.
The NIS is the largest publicly available, all-payer inpatient database in the United States. Encompassing data from over a thousand hospitals, the database records approximately eight million hospital stays per year. Furthermore, methods may be employed to gain national weighted estimates to be analyzed. In total, NIS comprises a stratified sample for nearly 20% of US community hospitals. The NIS database is de-identified and contains patient and hospital demographics, and information pertaining to billing, diagnostics, and procedures. Additional information regarding NIS may be found here.
The SID contains approximately 97% of all inpatient discharge records from community hospitals. Medicare, Medicaid, privately insured, and uninsured patients are all included. The database is widely used by researchers and policymakers to compare data, conduct market research, and analyze trends in healthcare utilization and outcomes. It contains more than one hundred clinical and non-clinical variables, such as principal and secondary diagnoses and procedures, admission and discharge status, patient demographics, total charge, and length of stay, with built-in safeguards to prevent identification of any particular patient, provider, or hospital. Additionally, a researcher can analyze the data to identify readmissions. Additional information regarding SID may be found here.