Lazzara 1991.

Methods	RCT (allocation by random‐number tables) in elderly nursing home population with 6‐month follow‐up.
Participants	Nursing home residents at risk of pressure ulcers (Norton score > 15). 9/66 subjects had pressure ulcers on entry to the study.
Interventions	1. Air‐filled (SofCare) overlay (33 randomised; 2 ulcer on admission; 10/31 developed a new one). 2. Gel mattress (33 randomised; 7 ulcer on admission; 8/26 developed a new one).
Outcomes	Grade 2 or greater ulcers: 1. Air overlay: 16% (5/31); 2. Gel mattress: 15% (4/26).
Notes	Interventions not well described. Of the 74 who entered the study, only those who participated for 4‐6 months were included in the analysis (total of 66). 19 patients died and were excluded from the analysis, but these might be at highest risk. It was difficult to maintain inflation of the air overlay; it also punctured easily. During the trial, 110 air overlays were used for 76 patients. Gel mattress was heavy.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	Low risk	Random‐number table.
Allocation concealment (selection bias)	Unclear risk	Not reported.
Blinding (performance bias and detection bias) Pressure ulcer incidence	Unclear risk	Blinding not reported: “patients in both study groups were assessed by the same researcher for the presence of pressure ulcer development over areas of bony prominence”.
Incomplete outcome data (attrition bias) All outcomes	Unclear risk	Nineteen participants died during the 6‐month study; individuals participating for 4‐6 months were included in the data analysis, although exact numbers included were not reported.
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.