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:…1964) and increases with age in in neurological diseases whereas others found it in non-complex sphingolipids as well but with a 50:50 ratio (Rosenberg & Stern 1966, Schwarz et al. 1967) for both chain lengths or even a much lower
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