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Table 2 Operational mattress definitions

	Alternating pressure mattress overlay	Alternating pressure mattress replacement
Alternating cell height minimum	3.5 inches/8.5 cm	8 inches/19.6 cm
Alternating Cell Height maximum	5 inches/12.25 cm	12 inches/29.4 cm
Cell Cycle Time	7.5–30 minutes	7.5–30 minutes
Cell Cycle	1 in 2 or 1 in 3 or 1 in 4	1 in 2 or 1 in 3 or 1 in 4

The Overlay and Replacement mattresses are easy to distinguish, if by no other feature, then by their maximum height. This requires little explanation to busy ward teams although their physical size would no doubt bring some difficulties. Nevertheless, elderly patients may be more likely to fall from the bed with the thicker mattress, which may cause some concern.

In most situations, the interventions are likely to be more complex than a choice of mattresses and hence need a detailed description. An example of part of a more complicated intervention is the concurrent therapy component of the concurrent chemo‐radiotherapy and adjuvant chemotherapy of protocol SQNP01 (1997) for non‐metastatic patients with nasopharyngeal cancer. For those familiar with the disease and its management in this single centre trial, the tabular format highlights the main components of what is involved.