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

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