Perhaps you should try some spelling variations:
"linkage groups" » "linkage group"
"life groups" » "lie groups", "like groups", "life group"
"link groups" » "risk groups", "disk groups", "online groups"
"sink groups" » "using groups", "risk groups", "disk groups"

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: (("life groups") OR ((("linkage groups") OR ((((("link groups") OR ("linear groups"))) OR ("sink groups")))))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"life groups\") or (((\"linkage groups\") or (((((\"link groups\") OR (\"linear groups\"))) or (\"sink groups\"))))))".
• Removing quotes may allow a broader search: (( life groups ) or ((( linkage groups ) or ((((( link groups ) OR ( linear groups ))) or ( sink groups )))))).
• Adding a wildcard symbol may retrieve word variants: (("life groups") or ((("linkage groups") or ((((("link groups") OR ("linear groups"))) or ("sink groups"))))))*.