Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:(4) Severe oxygenation failure: PaO2/FIO2 of less than 150 mmHg or intrapulmonary shunt 20% or greater while on positive end-expiratory pressure (PEEP) of 12 cm H2O or greater, with PAOP of less than 18 mmHg, and chest roentgenograph with infiltrates.
Category: