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:Still, it has been demonstrated that the psychological traits internal versus external locus of control are possible to affect by intervention among post-myocardial infarction patients at the level of secondary prevention [29] as well as health-related behaviours at the level of primary intervention [30].
Category: