Perhaps you should try some spelling variations:
"improving national performance series." » "improving regional performance series.", "improving operational performance series.", "improving educational performance series.", "improving national performance studies.", "improving national performance size.", "improving maternal performance series.", "improving natural performance series.", "improve national performance series."
"improving mental performance series." » "improving mental performance studies.", "improving mental performance size.", "improving general performance series.", "improving senegal performance series.", "improving annual performance series.", "improving maternal performance series.", "improving fiscal performance series.", "improving gender performance series."

Try widening your search to All Fields.

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

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