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:The risk of death of PWA, as compared to non-PWA, was estimated using sex-and agestandardized mortality ratios (SMRs).17 SMRs were calculated as the ratio between the observed number of deceased PWA who had a specific can er to the expected one, estimated on the basis of sex- and age-specific (quinquennia) mortality rates among non-PWA, multiplied by the person-years at risk among PWA.
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