Author study design quality country	Aim	Informants/data material	Main findings	Themes
Aiken et al. (2014) [31] Retrospective observational study Moderate Belgium, England, Finland, Ireland, The Netherlands, Norway, Spain, Sweden, Switzerland	To determine whether differences in patient to nurse ratio and nurses educational qualifications were associated with hospital mortality after common surgical procedures.	Discharge data from 422.730 patients 50 years or older, who underwent surgery and survey of 26.516 Nurses	Each increase of one patient per nurse is associated with a 7% increase in the likelihood of a surgical patient dying within 30 days of admission, whereas each 10% increase in the percent of bachelor degree nurses in hospital is associated with a 7% decrease in this likelihood.	Direct consequences: mortality
Aiken et al. (2002) [14] Cross-sectional analyses Moderate USA, California	To determine the association between the patient-to-nurse ratio and patient mortality, failure to rescue among surgical patients, and factors related to nurse retention	Data from 10.184 staff nurses surveyed, 232.342 general, orthopedic and vascular surgery patients and administrative data from 168 general hospitals	Each additional patient per nurse was associated with a 7% increase in the likelihood of dying within 30 days of admission and a 7% increase in the odds of failure to rescue (Patient and hospital characteristics were adjusted).	Direct consequences: mortality
Al-Kandari & Thomas (2009) [51] Cross-sectional survey Moderate Kuwait	To identify the perceived adverse patient outcomes related to nurse’ workload	780 registered nurses working in medical and surgical wards of five governmental hospitals in Kuwait.	Five major perceived adverse patient outcomes: 1) complaints from patient and their families, 2) patient received a late dose or missed a dose of medication, 3) discovering pressure ulcer, 4) wound infection and 5) infection on the site of IV cannula.	Indirect consequences: Poor basic quality of care
Amarvadi et al. (2000) [32] Observational cohort study Moderate USA	To determine if having a 1:2 rather than a 1:3 or more night-time nurse-to-patient ratio (NNPR) in the intensive care unit affected hospital mortality	Adult patients who had esophageal resection in Maryland 1994 to 1998 (366 patients)	There were no significant difference in the risk of in-hospital mortality between patients with a NNRP > 1:2 (Night time nurse-to-patient ratio > one nurse caring for one or two patients) and those with a NNRP < 1:2 (Nurse caring for three or more patients) Patients with a NNPR < 1:2 had an increased risk of reintubation, pneumonia and sepsis	Direct consequences: Patient harm
Carthonet al. (2012) [33] Cross-sectional survey Moderate USA, Pennsylvania	To determine the association between nurse staffing and postsurgical outcomes for older black adults, including 30-day mortality and failure to rescue.	548.397 patients ages 65 and older, undergoing general, orthopedic or vascular surgery	One additional patient in the average nurse’s workload was associated with higher odds of 30 day mortality for all patients. Odds of failure to rescue were higher for patient in settings with poorer nursing staffing	Direct consequences: Mortality
Cho et al. (2015) [52] Cross sectional study Moderate South Korea	To examine the relationship of nurse staffing levels and work environment with patient adverse events	4864 nurses Data from 58 hospitals Discharge data from 113,426 Patients	A large number of patients per nurse were significantly associated with a greater incidence of administration of wrong medication or dose, pressure ulcers and patient falls with injury.	Indirect consequences: medication errors and patient harm.