Andersen 1982.

Study characteristics
Methods	Study objective: to observe "the development of pressure sores in risk‐patients nursed on these mattresses [water‐mattresses and alternating pressure air‐mattresses and compare] the results with a similar group of patients nursed on ordinary hospital mattresses" Study design: randomised controlled trial Study grouping: parallel group Duration of follow‐up: 10 days Number of arms: 3 Single centre or multi‐site: single centre Study start date and end date: not described Setting: hospital
Participants	Baseline characteristics Inclusion criteria: patients with acute conditions and a risk score of 2 or more (i.e. at risk) Exclusion criteria: "those who already had pressure sores" Sex (M:F): 60:101 in control; 73:93 in air; 73:82 in water Age (years): distribution of participants' ages described Baseline skin status: all at risk according to the risk score used by the authors; free of ulcers Group difference: no difference between groups according to age, sex, body weight or risk score Total number of participants: not described; n = 482 available Unit of analysis: individuals Unit of randomisation (per patient): individuals
Interventions	Intervention characteristics Alternating‐pressure air‐mattress Description of interventions: "2 metres long and consists of longitudinal air tubes connected in two separate series ... Each of the two series is inflated and deflated alternately by an electrically driven pump, providing sufficient air‐pressure to support the patient ... for about 5 minutes. The mattress is placed on top of an ordinary hospital mattress" NPUAP S3I classification: powered, alternating pressure (active) air surface Co‐interventions: not described Number of participants randomised: not described; 166 available Number of participants analysed: n = 166 Water mattress Description of interventions: "a box‐shaped container 200 by 90 by 15 cm ... filled with lukewarm water and placed on top of a hospital mattress ... to keep the patient afloat" NPUAP S3I classification: non‐powered, reactive water‐filled surface Co‐interventions: not described Number of participants randomised: not described; 155 available Number of participants analysed: n = 155 Ordinary hospital mattresses Description of interventions: not described NPUAP S3I classification: standard hospital surface Co‐interventions: not described Number of participants randomised: not described; 161 available Number of participants analysed: n = 161
Outcomes	Proportion of participants developing a new pressure ulcer Outcome type: binary Time points: 10 days Reporting: partially reported Measurement method (e.g. scale, self‐reporting): researchers‐assessed; ulcer classification system not described Definition (including ulcer stage): using bullae, black necrosis and skin defect as evidence of pressure sores; stage of ulcer not described Dropouts: not described Notes (e.g. other results reported): 21 patients in control versus 7 patients in water‐mattress versus 7 patients in air‐mattress Time to pressure ulcer development Reporting: not reported Support‐surface‐associated patient comfort Reporting: not reported All reported adverse events using allocated support surfaces Reporting: not reported Health‐related quality of life (HRQOL) Reporting: not reported Cost‐effectiveness Reporting: not reported Notes: water‐mattress price GBP (pounds sterling) 20; alternating‐pressure air‐mattress price GBP 200 Outcomes that are not considered in this review but reported in trials: 1. "opinions on mattresses" described as "the acceptability of the mattress" and rated as the numbers of staff satisfied and the numbers of patients satisfied with different mattresses.
Notes
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Unclear risk	Quote: "Six hundred patients at risk for pressure sores were randomised in either a control group or one of two experimental groups ... They were allotted to one of the three groups ..." Comment: method of randomisation was not reported
Allocation concealment (selection bias)	Unclear risk	Comment: no information provided
Blinding of participants and personnel (performance bias) All outcomes	Unclear risk	Outcome group: primary outcome (i.e. the only outcome) Comment: no information provided
Blinding of outcome assessment (detection bias) All outcomes	High risk	Outcome group: primary outcome (i.e. the only outcome) Quote: "One of us [note: study's authors] assessed the condition of the skin ..." Comment: appears to have no blinding, and the pressure ulcer incidence outcome measurement is likely to be influenced by lack of blinding.
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	Outcome group: primary outcome (i.e. the only outcome) Quote: "Six hundred patients at risk for pressure sores were randomised ..." Quote: "Among the 600 risk‐patients ... 118 dropped out during the first 24 hours before the first dermatologic inspection. This did not impair randomization." Quote: "The groups remained comparable throughout the 10‐day study period" Comment: unclear risk of bias was judged because authors claimed that randomisation was not impaired though the proportion of missing data was high and no reasons for missing data were provided.
Selective reporting (reporting bias)	Low risk	Comment: the study protocol is not available but it is clear that the published reports include all expected outcomes, including those that were pre‐specified.
Other bias	Low risk	Comment: the study appears to be free of other sources of bias.