Gebhardt 1996.

Methods	Trial allocation by hospital number. Two systems: patients were automatically placed on the low‐cost mattress within the allocated system. Patients who deteriorated or experienced persisted erythema were transferred to a medium‐cost mattress. If deterioration continued they were placed on the highest‐cost mattress, or transferred to the alternate group, if appropriate.
Participants	Patients in ICU with a Norton score <13, who had been in the unit for < 3 days, with no pressure ulcers.
Interventions	1. Alternating‐pressure air mattresses (shallow small cell overlays, medium depth large cell overlays, deep mattresses and deep pulsating low‐air‐loss beds) (n = 23). 2. Constant low‐pressure supports (fibre overlays, foam mattresses/overlays, static air overlays, gel overlay, water overlay, bead overlay, low‐air‐loss mattresses, static air overlay, low‐air‐loss beds and air‐fluidised bead beds) (n = 20).
Outcomes	1. Support provided. 2. Pressure ulcer development: Alternating pressure group (n=0 participants), Constant low‐pressure group (n=8 participants of which grade 2 (n=4), grade 3 (n=2), both grade 2 and 3 (n=2)) 3. Cost: Low cost (less than £500) n=22 (n=1shallow small cell overlay, n=5 fibre overlays, n=4 medium‐depth large‐cell overlays, n=6 foam mattresses/overlays, n=3 static air overlays, n=1 gel overlay, n=1 water overlay, n=1 bead overlay) Medium cost (£500‐ £5000) n=4 (n=2 deep mattresses, n=1 low‐air‐loss mattresses, n=1 static air overlays) High cost (Greater than £5000) n=6 (n=2 deep pulsating low‐air‐loss beds, n=2 low‐air‐loss beds, n=2 air fluidised bead beds)
Notes	No ITT analysis. Mechanical unreliability and poor management of alternating‐pressure supports was a problem.
Risk of bias
Bias	Authors' judgement	Support for judgement
Random sequence generation (selection bias)	High risk	Patients allocated "according to the final digit of their hospital number (even to alternating pressure, odd to constant low pressure supports)."
Allocation concealment (selection bias)	High risk	Patients allocated "according to the final digit of their hospital number (even to alternating pressure, odd to constant low pressure supports)."
Blinding (performance bias and detection bias) Pressure ulcer incidence	Unclear risk	Not reported.
Incomplete outcome data (attrition bias) All outcomes	Low risk	Reasons for exclusion/attrition given.
Selective reporting (reporting bias)	Unclear risk	Pre‐specified aims not reported.  Incidence of pressure ulcers as well as mean cost per type of mattress provided.
Free of other bias ‐ were groups similar at baseline regarding the most important prognostic indicators?	Low risk	As shown in Table 2.
Free of other bias ‐ was the timing of the outcome assessment similar in all groups?	Unclear risk	“Patients were visited four times weekly by the research nurse”, however, “patients were taken out of the trial after three months, or if their condition improved so that they were no longer at risk of developing pressure sores, if they were discharged or transferred to another ward or hospital, or if they died”.