A nurse staffing measure was calculated as the mean patient load across all staff registered nurses who reported having responsibility for at least 1 but fewer than 20 patients on the last shift they worked, regardless of the specialty or shift (day, evening, night) worked. This measure of staffing is superior to those derived from administrative databases, which generally include registered nurse positions that do not involve inpatient acute care at the bedside. Staffing was measured across entire hospitals because there is no evidence that specialty-specific staffing offers advantages in the study of patient outcome17 and to reflect the fact that patients often receive nursing care in multiple specialty areas of a hospital. Direct measurement also avoided problems with missing data common to the AHA's Annual Survey of hospitals, which imputed staffing data in 1999 for 20% of Pennsylvania hospitals.