Perhaps you should try some spelling variations:
"social exact model" » "social effect model", "social etats model", "social et model"
"social needs model" » "special needs model", "social norms model", "social needs more"
"social net model" » "social news model", "social na model"

Try widening your search to All Fields.

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: (("social exact model") OR ((("social net model") OR ("social needs model")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"social exact model\") or (((\"social net model\") OR (\"social needs model\"))))".
• Removing quotes may allow a broader search: (( social exact model ) or ((( social net model ) OR ( social needs model )))).
• Adding a wildcard symbol may retrieve word variants: (("social exact model") or ((("social net model") OR ("social needs model"))))*.