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 few available studies of longitudinal changes in BMD in patients with HIV infection have shown conflicting results [13, 14, 18–23], which may relate to the heterogeneous use of antiretrovirals or, more importantly, to the timing of the first DXA measurements in relation to initiation of treatment.
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