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:Similar to our results, concentrations of 3.15 ngmL 1, 31.5 ngmL 1 and 315 ngmL 1 of progesterone added during the culture phase of bovine in vitroproduced embryos did not impact on the proportion of zygotes reaching the blastocyst stage in a recent study (Clemente et al. 2009).
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