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:The Error Duration Model of Parke (1999) is essentially a discrete time analogue of the aforementioned model. However, the article contributes to the economics literature by formulating and interpreting the model in economic terms. The model to be presented below is an extension of Parke’s (1999) Error Duration Model, and the exposition follows that of Parke (1999) for ease of comparison.
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