Perhaps you should try some spelling variations:
"improving central performance contract." » "improving central performance contracts.", "improving control performance contract.", "improving general performance contract.", "improving contract performance contract.", "improving mental performance contract.", "improving federal performance contract.", "improving neural performance contract."
"improving high performance series." » "improving high performance studies.", "improving high performance size.", "improving higher performance series.", "improving hiv performance series.", "improving birth 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 central performance contract.") OR ("improving high performance series."))~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"improving central performance contract.\") OR (\"improving high performance series.\"))*".
• Removing quotes may allow a broader search: (( improving central performance contract. ) OR ( improving high performance series. ))*.