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:47 I construct estimates of lifetime solar generation based 46This calculation of external benefits builds on Gillingham and Tsvetanov (2017). 47PVWatts is developed by the National Renewable Energy Laboratory and accessible via http://pvwatts.
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