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:…which is lower than previous estimates of 30-80%, probably results from our use of a national data source which does not restrict the cases to children with CP who attend specialty practices or other sites of research (Graham & Rutter, 1968a; Houcade & Pasrette, 1984; McDonald, 1987; Rosenbloom.
Category: