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:It is also in agreement with those of Stansley et al. (1997) who obtained high mortality in R. palustris tadpoles, exposed from the egg stage for 10 days to a total mean lead concentration of 1.670 and 0.944 mg Pb/l.
Category: