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:Equation 6 is frequently used to calculate CD from 3D wind measurements based on knowledge of PAD and estimations for Pm (Amiro 1990; Kerzenmacher and Gardiner 1998; Mahrt et al. 2001; Pinard and Wilson 2001; Cescatti and Marcolla 2004).
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