The National Inpatient Sample (NIS) is a large database of hospital inpatient stays in the United States. It is part of the Healthcare Cost and Utilization Project (HCUP) and is maintained by the Agency for Healthcare Research and Quality (AHRQ). The NIS includes data on patient demographics, diagnoses, procedures, outcomes, and hospital characteristics for millions of hospitalizations each year. Researchers use the NIS to analyze trends in healthcare utilization, outcomes, and costs, as well as to study specific diseases or conditions. It is a valuable resource for understanding the current state of healthcare in the United States and identifying areas for improvement.