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:Both of the studies conducted by Dhandu and Angel (2003) and Yan et al. (2001) also focused on broilers older than those used in this experiment, a developmental stage where broilers are less sensitive to nPP (Nelson et al., 1990), which may explain the difference in results obtained.
Category: