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:Biological neurons in vivo show a substantial variability in the exact timing of action potentials to identical stimulus presentations (Calvin & Stevens, 1968; Softky & Koch, 1993; Shadlen & Newsome, 1998).
Category: