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:…813 ± 82 831 ± 55
a aa, amino acids. b Based on within-vertebrate divergence times. c Based on animal–fungus divergence times.
invertebrate divergence time is more reliable than theirs, because mitochondrial genes they used are not evolving in a clocklike fashion in vertebrates (Nikoh et al. 1997).
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