Table 1.

PICOS framework

PICOS	Inclusion criteria	Exclusion criteria
Population:	Adult patients (aged over 18 years) in acute care hospital wards. No restrictions were imposed regarding hospital or ward size, teaching status or sector.	Participants in peri‐operative, maternity, paediatrics (aged < 18 years), mental health, substance abuse, nursing home or palliative care environments
Intervention:	Studies reviewing a particular type of nursing skill mix or skill mix level and/or compared them with a different (e.g., baseline) skill mix type or level were included in the review.	Studies examining patient‐to‐nurse ratios were excluded, as ratios had been covered by another review.
Comparator:	A different nursing skill mix level or no comparator.
Outcome:	Patient outcomes (sensitive to nursing care) such as mortality, deep vein thrombosis, sepsis, urinary tract infection, pressure injuries, pneumonia, upper gastro‐intestinal bleeding, shock/cardiac arrest, central nervous system complications, surgical wound infections, pulmonary failure, physiologic/metabolic derangement, or any others identified (and tested) by researchers as outcomes potentially sensitive to nursing care
Study design:	Observational/descriptive (includes cross‐sectional, prospective/cohort studies, case‐control studies) and experimental (includes experimental [RCT], quasi‐experimental [time‐series etc.]), and mixed methods designs were included if the quantitative component was relevant to the research question.	Qualitative studies