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:All ‘don’t know’ and ‘unsure’ responses (Table 1) were classed as missing values in the dataset and Maximum Likelihood (ML) imputation was used to estimate replacement values for all missing data [52,53].
Category: