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:Mothers with incomplete socio-demographic or labor data (n = 36), caesarean sections (n = 42) or exceptionally long labor (> 24 hours, n = 1) were excluded because labor has been reported to influence lymphocyte distribution [15], leaving 1,397 subjects for this study.
Category: