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:According to Cochrane Database of systematic Reviews, all of these three drugs are efficacious for mild to moderate AD (Birks 2006) though the symptomatic relief provided byAChE inhibition therapy is weak (Lleó et al. 2006) and fails to reverse disease progression (so-called disease-modifying effect).
Category: