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:SEM is a powerful tool that allows for the relative effect of many explanatory factors on a response variable to be determined, even if many of the explanatory factors are correlated (Grace 2006; Schumacker and Lomax 2004).
Category: