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:To avoid difficulties in multiple-regression analysis resulting from highly interdependent traits (Mitchell-Olds and Shaw 1987), a single size index was calculated for each individual by taking the product of 2003 stem height and total leaf area.
Category: