Perhaps you should try some spelling variations:
"improving human performance contract." » "improving human performance contracts.", "improving loan performance contract.", "improving thermal performance contract.", "improving chain performance contract."
"improving workers performance series." » "improving worker performance series.", "improving work performance series.", "improving markets performance series.", "improving workers performance studies.", "improving workers performance size.", "improving farmers performance series.", "employing workers performance series."

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: (("improving human performance contract.") or ("improving workers performance series."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving human performance contract.") OR ("improving workers performance series."))*.
• Removing quotes may allow a broader search: (( improving human performance contract. ) or ( improving workers performance series. ))*.