Perhaps you should try some spelling variations:
"improving voluntary performance risk." » "improving country performance risk.", "improving entry performance risk.", "improving budgetary performance risk.", "improving voluntary performance size.", "improving voluntary performance based.", "improving voluntary performance _."
"improving city performance series." » "improving city performance metrics.", "improving city performance reviews.", "improving city performance review.", "improving firm performance series.", "improving high performance series.", "improving key performance series.", "improving its performance series.", "improving equity 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 voluntary performance risk.") OR ("improving city performance series."))~.
• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"improving voluntary performance risk.\") OR (\"improving city performance series.\"))".
• Removing quotes may allow a broader search: (( improving voluntary performance risk. ) OR ( improving city performance series. )).
• Adding a wildcard symbol may retrieve word variants: (("improving voluntary performance risk.") OR ("improving city performance series."))*.