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:Parsing accuracy is measured in the experiments by three different metrics: • Attachment score (AS): The proportion of tokens (excluding punctuation) that are assigned the correct head (or no head if the token is a root) (Eisner, 1996; Collins et al., 1999).
Category: