Perhaps you should try some spelling variations:
"improving thermal performance series." » "improving thermal performance metrics.", "improving thermal performance reviews.", "improving thermal performance review.", "improving maternal performance series.", "improving rural performance series.", "improving human performance series."
"improving mental performance series." » "improving mental performance metrics.", "improving mental performance reviews.", "improving mental performance review.", "improving senegal performance series.", "improving annual performance series.", "improving entry performance series.", "improving maternal performance series.", "improving social performance series.", "improving rural 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 thermal performance series.") or ("improving mental performance series."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving thermal performance series.") OR ("improving mental performance series.")).
• Removing quotes may allow a broader search: (( improving thermal performance series. ) or ( improving mental performance series. )).
• Adding a wildcard symbol may retrieve word variants: (("improving thermal performance series.") or ("improving mental performance series."))*.