Perhaps you should try some spelling variations:
"improving country performance series." » "improving country performance metrics.", "improving country performance reviews.", "improving country performance review.", "improving voluntary performance series.", "importing country performance series.", "borrowing country performance series."
"improving poor performance series." » "improving poor performance metrics.", "improving poor performance reviews.", "improving poor performance review.", "improving export performance series.", "improving sector performance series.", "improving _ performance series.", "improving power performance series.", "improving food performance series.", "improving indoor 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 country performance series.") or ("improving poor performance series."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving country performance series.") OR ("improving poor performance series."))*.
• Removing quotes may allow a broader search: (( improving country performance series. ) or ( improving poor performance series. ))*.