Perhaps you should try some spelling variations:
model literature » models literature, model literatur, modern literature
del literature » abel literature, fuel literature, al literature
tests safety » test safety, guests safety, stents safety
studies die » studie die, studies does, studies dose
studien die » studie die, studien der

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: tests safety ((studies die) OR (studien die)) filosofia ((del literature) OR (model literature)).
• The words AND, OR and NOT may confuse the search; try adding quotes: "tests safety ((studies die) or (studien die)) filosofia ((del literature) OR (model literature))".
• Adding a wildcard symbol may retrieve word variants: tests safety ((studies die) or (studien die)) filosofia ((del literature) OR (model literature))*.