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:(4)
Since estimating these conditional probabilities is impractical, following an earlier approach (Blaylock and Allen 2003), Mott et al. (2006) defined two n-gram models based on a Markov assumption that an observation !! depends only on the goal ! and a limited window of preceding observations.
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