Patients who have to stay in the hospital again within 30 days of their hip or knee replacement surgery. Lower is better.
Going back to the hospital within 30 days of getting out of the hospital can mean people aren't getting the right care after they leave the hospital.
Hospital 30-Day Readmission Rates after Hip/Knee Surgery: http://www.medicare.gov/hospitalcompare/Data/30-day-measures.html