Previous research has shown that patients admitted to the hospital on the weekend are at increased risk for worse outcomes, including mortality. This retrospective study examined more than 3 million emergency admissions to 140 hospital trusts in England between April 2013 and February 2014. Patient arrival times were recorded by day of the week and nighttime versus daytime. Using administrative data and standard risk adjustment, mortality rates were higher for patients arriving during the week on Wednesday and Thursday nights. Risk-adjusted mortality rates were also found to increase for patients arriving over the weekend from daytime on Saturday through nighttime on Sunday. However, when researchers adjusted for arrival by ambulance, higher mortality was statistically significant only for those patients arriving at the hospital during the day on Sunday. Investigators suggest that prior research supporting the weekend effect is overly reliant on administrative data, which may not accurately characterize illness severity. It is often debated whether the weekend effect could be due to factors related to the system of care (i.e., reduced staffing on weekends) or patient factors (i.e., increased severity of illness of patients admitted on the weekend). An Australian study sought to answer this question and found that certain diagnoses appeared to be associated with higher mortality for weekend admissions, largely due to health system factors.