Perhaps you should try some spelling variations:
travel surveys » travel survey, level surveys, travel success
level studies » legal studies, event studies, local studies
panel surveys » panel survey, phone surveys, level surveys
tie surveys » site surveys, life surveys, line surveys
cold level » color level, word level, second level

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: harvard cold level studies ((tie surveys) OR (((panel surveys) OR (travel surveys)))).~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "harvard cold level studies ((tie surveys) or (((panel surveys) OR (travel surveys)))).~".
• Adding a wildcard symbol may retrieve word variants: harvard cold level studies ((tie surveys) or (((panel surveys) OR (travel surveys)))).*.