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:Letters
 The Role of Ocular Perfusion Pressure in Glaucoma Cannot Be Studied With Multivariable Regression Analysis Applied to Surrogates
 Ocular perfusion pressure (OPP) is not easily directly measurable, and the difference between blood pressure and intraocular pressure (IOP) has been suggested as a simple surrogate of OPP.1 We write in relation to publications in this journal2,3 and elsewhere4 that have reported a significant association between OPP surrogates and glaucoma.
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