Perhaps you should try some spelling variations:
"improving national performance series." » "improving regional performance series.", "improving operational performance series.", "improving educational performance series.", "improving natural performance series.", "improve national performance series.", "improving national performance studies.", "improving national performance size."
"improving maternal performance risks." » "improving materials performance risks.", "improving external performance risks.", "improving managerial performance risks.", "improve maternal performance risks.", "improving water performance risks.", "improving maternal performance risk."

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