Perhaps you should try some spelling variations:
"improving voluntary performance _." » "improving country performance _.", "improving entry performance _.", "improving budgetary performance _."
"improving city performance review." » "improving city performance risk.", "improving city performance evidence.", "improving firm performance review.", "improving high performance review.", "improving key performance review.", "improving its performance review.", "improving equity performance review.", "improving birth performance review."

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

• Performing a fuzzy search may retrieve terms with similar spellings: (("improving voluntary performance _.") or ("improving city performance review."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving voluntary performance _.") OR ("improving city performance review.")).
• Removing quotes may allow a broader search: (( improving voluntary performance _. ) or ( improving city performance review. )).
• Adding a wildcard symbol may retrieve word variants: (("improving voluntary performance _.") or ("improving city performance review."))*.