Perhaps you should try some spelling variations:
"improving maternal performance metrics." » "improving materials performance metrics.", "improving external performance metrics.", "improving national performance metrics.", "improving maternal performance measures.", "improve maternal performance metrics.", "improving water performance metrics."
"improving human performance series." » "improving human performance studies.", "improving human performance size.", "improving loan performance series.", "improving thermal performance series.", "improving chain 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 human performance series.") or ("improving maternal performance metrics."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving human performance series.") OR ("improving maternal performance metrics.")).
• Removing quotes may allow a broader search: (( improving human performance series. ) or ( improving maternal performance metrics. )).
• Adding a wildcard symbol may retrieve word variants: (("improving human performance series.") or ("improving maternal performance metrics."))*.