Perhaps you should try some spelling variations:
de litteratura » de litterature, e letteratura, da literatura
model studien » modell studien, model studies, models studies
de literatura » de litterature, la literatura, y literatura
test model » test models, trust model, cost model

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: test model studien _ filosofia ((de litteratura) OR (de literatura)).
• Adding a wildcard symbol may retrieve word variants: test model studien _ filosofia ((de litteratura) or (de literatura))*.