Perhaps you should try some spelling variations:
"improving country performance series." » "improving country performance studies.", "improving country performance size.", "improving voluntary performance series.", "importing country performance series.", "improving community performance series."
"improving thermal performance series." » "improving thermal performance studies.", "improving thermal performance size.", "improving external performance series.", "improving animal performance series.", "improving erm 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 country performance series.") or ("improving thermal performance series."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving country performance series.") OR ("improving thermal performance series.")).
• Removing quotes may allow a broader search: (( improving country performance series. ) or ( improving thermal performance series. )).
• Adding a wildcard symbol may retrieve word variants: (("improving country performance series.") or ("improving thermal performance series."))*.