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:As the rate of periapical abscesses in the CH group was the same as in the induction study [Cleaton-Jones et al., 2002] but that in the ZOE teeth was a quarter of the induction rate, there is a suggestion that ZOE is superior to CH. The higher success rate may also have been influenced by filling to the apex only, as Holan and Fuks [1993] showed clearly that overfilling with ZOE is detrimental to periapical healing. Tchaou et al. [1996] exposed standard bacterial cultures to 10 root filling materials or combinations of these.
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