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 review." » "improving thermal performance risk.", "improving thermal performance evidence.", "improving external performance review.", "improving animal performance review.", "improving erm performance review."

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

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