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:Expression values for PIF3 were calculated using the equation 2-ΔΔCt, and 18S rRNA served as the control (Livak and Schmittgen, 2001; Pfaffl, 2001).
Category: