Alternatively, the literature can be in order of importance whereby some research is relevant and more salient (lending itself to description using the present perfect and present tense), while other research may now be superceded or may need to be made less important, and thus contrasted by using the simple past tense. This contrast, however, is taking place within your thesis which is in the present, e.g.
Maclean (1986) classified intangible assets using ..... Sorenson, however, has suggested that this taxonomy is no longer appropriate and that....
Fellows and Markham, in their study of adolescence substance abuse in Britain, found that teenagers preferred to ..... Of the 300 respondents interviewed in the course of their study, 73% indicated that..... This finding is contradicted by research in Sweden by Sorensen, who have found that...
We can also use tense in a pragmatic way, that is, to generalise information or to render it applicable to a specific occasion:
'According to Smith (1993) adolescent substance abuse is highest in cities' (a generally accepted fact)
'According to Smith (1993) adolescent substance abuse was highest in cities' (the results of specific research which may have been superseded)
Alternatively, tense might also be controlled to make some information more salient (eg, with the present perfect) than other information (eg with the simple past):
Maclean (1986) classified intangible assets using ........ Sorenson, however, has suggested that this taxonomy is no longer appropriate and that.... '
Several studies have reported similar findings with older sample populations.....