Perhaps you should try some spelling variations:
de litteratura » de litterature, de literatura, e letteratura
e literatura » de literatura, y literatura, e literatur
test modell » tekst modell, test model, text modell
test models » trust models, cost models, testing models

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 modell) OR (test models)) studien _ filosofia ((de litteratura) OR (e literatura)).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((test modell) OR (test models)) studien _ filosofia ((de litteratura) or (e literatura))".
• Adding a wildcard symbol may retrieve word variants: ((test modell) OR (test models)) studien _ filosofia ((de litteratura) or (e literatura))*.