Nixon 2006.

Methods	RCT with 30‐day follow‐up twice weekly, and a further 30‐day follow‐up once weekly.
Participants	Recruited from 11 hospitals. Patients admitted as acute or elective cases. Eligible patients aged ≥ 55, expected to stay for at least 7 days, with either limited activity or mobility (Braden scale activity and mobility score of 1 or 2), or an existing pressure ulcer of grade 2. Elective surgical participants without limited activity or mobility were eligible if the mean LOS for surgery was at least 7 days and they were expected to have Braden scale activity and mobility scores of 1 or 2 for at least 3 days postoperatively. Exclusion criteria: grade 3 or worse pressure ulcer on admission, planned admission to ICU after surgery, admitted to hospital more than 4 days before surgery, slept at night in a chair, weighted > 140 kg or < 45 kg (as per mattress specifications). Participants were well matched at baseline.
Interventions	1. Alternating‐pressure overlay (n = 990): alternating cell height minimum 8.5cm, max 12.25cm; cell cycle time 7.5‐30 minutes. 2. Alternating‐pressure mattress (n = 982): alternating cell height min 19.6cms, max 29.4cms; cell cycle time 7.5‐30 minutes. Intervention was allocated within 24 hrs of admission.
Outcomes	Number of participants with incidence pressure ulcer grade 2 and above (unblinded outcome assessment): 1. Overlay: 11% (106/989); 2. Mattress: 10% (101/982). Patient acceptability: requests for mattress change: 1. Overlay: 23% (230/989); 2. Mattress: 19% (186/982). Healing of existing pressure ulcers: 1. Overlay: 34% (20/59); 2. Mattress: 35% (19/54). Cost of treatment (GBP): 1. Overlay: Sterling 6793.33; 2. Mattress: Sterling 6509.73. Mean difference in time to pressure ulcer (grade 2 or higher) development (days). Participants in mattress group took 10.64 days longer to develop pressure ulcer than overlay group.
Notes	1 participant was recruited to the trial twice (group 1) and was excluded from analysis. Factors that had a significant effect on the proportion of people developing a new pressure ulcer were admission for an acute condition, the presence of a wound skin trauma or non‐blanching erythema on any site at baseline, age, haemoglobin level and diabetes. The authors stated that differences in health benefits and total costs for hospital stay between alternating‐pressure mattresses and alternating‐pressure overlays were not statistically significant. However, a cost effectiveness acceptability curve indicated that on average alternating‐pressure mattresses were associated with an 80% probability of cost saving compared with alternating‐pressure overlays.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Randomisation using a computer‐generated algorithm.
Allocation concealment (selection bias)	Low risk	"To maintain allocation concealment, the minimisation algorithm and subsequent treatment assignment was provided through an independent, central, secure 24‐hour randomisation automated telephone service by the Clinical Trials Research Unit (CTRU), University of Leeds".
Blinding (performance bias and detection bias) Pressure ulcer incidence	High risk	Stated, “owing to the nature of the mattresses under investigation, it was not possible to mask the randomised intervention to the patients participating in the trial, ward nursing staff or the CRNs conducting the skin assessments”.
Incomplete outcome data (attrition bias) All outcomes	Low risk	ITT analysis conducted.
Selective reporting (reporting bias)	Low risk	All pre‐specified outcomes reported.
Free of other bias ‐ were groups similar at baseline regarding the most important prognostic indicators?	Low risk	No important baseline differences.
Free of other bias ‐ was the timing of the outcome assessment similar in all groups?	Low risk	No other concerns.