Perhaps you should try some spelling variations:
"improving firm performance series." » "improving firm performance studies.", "improving firm performance size.", "improving firms performance series.", "improving air performance series.", "improving erm performance series."
"improving music performance evidence." » "improving public performance evidence.", "improving using performance evidence.", "improving seismic performance evidence.", "improving basic performance evidence.", "improvised music performance evidence.", "improving civic performance evidence."

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

• Performing a fuzzy search may retrieve terms with similar spellings: (("improving firm performance series.") or ("improving music performance evidence."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving firm performance series.") OR ("improving music performance evidence.")).
• Removing quotes may allow a broader search: (( improving firm performance series. ) or ( improving music performance evidence. )).
• Adding a wildcard symbol may retrieve word variants: (("improving firm performance series.") or ("improving music performance evidence."))*.