Perhaps you should try some spelling variations:
"improving national performance series." » "improving regional performance series.", "improving operational performance series.", "improving educational performance series.", "improving maternal performance series.", "improving natural performance series.", "improve national performance series.", "improving national performance studies.", "improving national performance size."
"improving thermal performance evidence." » "improving external performance evidence.", "improving animal performance evidence.", "improving erm performance evidence."

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 thermal performance evidence.") OR ("improving national performance series."))~.
• Removing quotes may allow a broader search: (( improving thermal performance evidence. ) or ( improving national performance series. ))~.
• Adding a wildcard symbol may retrieve word variants: (("improving thermal performance evidence.") or ("improving national performance series."))*.