Twenty-seven studies were included: 3 randomised controlled trials (RCTs), 7 prospective studies and 17 retrospective studies. The number of patients in the included studies was not reported.
The quality of the included studies varied considerably. Three of the studies were RCTs, two did not fulfil any of the validity assessment criteria, and in one study no validity assessment scores were reported. Eleven studies had used uniform methods of data collection, two had used methods of random sample to select hospitals for inclusion in the study, nine had used a sample of hospitals from dispersed geographic regions, 13 had included five or more hospitals in the study, seven were of a prospective design (excluding the 3 RCTs), nine had collected data for more than one year, and 17 had used statistical methods in the analysis.
Nursing autonomy (5 studies): 3 studies found an association between nursing autonomy and lower patient mortality rates. The remaining 2 studies found no significant association.
Nursing workload (10 studies): 8 studies found a correlation between nursing workload and patient mortality rates, but this was not consistent across the studies. Five studies found that greater nursing workload demands were associated with higher patient mortality rates, three found that increased workload was associated with lower patient mortality, and two found no association.
Inter-professional relations (10 studies): 6 studies found a significant positive association between nurse-physician relations and patient mortality, three found no association, and one had insufficient power to detect a difference. The two most rigorously conducted studies, produced disparate results.
Nursing management (6 studies): the one RCT that assessed the relationship between nursing care and patient mortality found no significant differences in mortality rates. Four other studies found lower rates associated with a diverse range of nursing management attributes, whilst one study found no association.
Nursing standards (3 studies): all 3 studies found a significant positive correlation between an attribute that represented a nursing standard and patient mortality. However, many of the case-mix and validity criteria were not fulfilled in the studies.
Professional development (4 studies): 3 studies found a significant negative association between nursing professional development and mortality. The fourth assessed the impact of a continuing education programme and found no significant association.
Mediating processes (3 studies): all 3 studies found lower mortality rates associated with favourable nurse-mediating processes.