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:When the outcome of the literature review was deemed not to qualify for inclusion (e.g., study size too small for stable age-specific counts, or reports focusing on mortality rather than incidence of CJD), the required agespecific incidence counts for 5-year age groups (cases and, sometimes, person-year denominators) were obtained upon request from three sources: firstly from authors reporting ALS incidence on the basis of National Registry data in Sweden [17,18]; secondly, from the Research and Surveillance Unit at the Edinburgh Unit (RGWill) for atCJD and vCJD in the UK, for the period 1993–2007; and thirdly, from the Spanish CJD Registry 1995–2013 for gCJD, sCJD and sRPNDd (Table 2).
Category: