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 based performance series." » "improving based performance metrics.", "improving based performance reviews.", "improving based performance review.", "improving bank performance series.", "improving care performance series.", "improving board performance series.", "improving basic performance series.", "improving shared performance series.", "improving water 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 based performance series."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving central performance contract.") OR ("improving based performance series.")).
• Removing quotes may allow a broader search: (( improving central performance contract. ) or ( improving based performance series. )).
• Adding a wildcard symbol may retrieve word variants: (("improving central performance contract.") or ("improving based performance series."))*.