Perhaps you should try some spelling variations:
"improving voluntary performance review." » "improving country performance review.", "improving entry performance review.", "improving budgetary performance review.", "improving voluntary performance risk.", "improving voluntary performance evidence."
"improving past performance series." » "improving past performance metrics.", "improving past performance reviews.", "improving past performance targets.", "improving plant performance series.", "improving port performance series.", "improving based performance series.", "improving basic performance series.", "improving asset performance series.", "improving land performance series."

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

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