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:Disadvantages include those described above for DNA (if this is the PCR target, although disadvantage d would not apply) or RNA (if reversetranscriptase PCR is used), plus the unknown yet potentially variable amplification efficiencies of different target sequences (Suzuki & Giovannoni, 1996).
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