Perhaps you should try some spelling variations:
"improving central performance contract." » "improving central performance contracts.", "improving contract performance contract.", "improving sectoral performance contract.", "improving entry performance contract.", "improving mental performance contract.", "improving critical performance contract.", "improving energy performance contract."
"improving high performance series." » "improving high performance metrics.", "improving high performance reviews.", "improving high performance review.", "improving firm performance series.", "improving _ performance series.", "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."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("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. ))*.