Perhaps you should try some spelling variations:
"improving voluntary performance evidence." » "improving country performance evidence.", "improving entry performance evidence.", "improving industry performance evidence."
"improving poor performance series." » "improving poor performance studies.", "improving poor performance size.", "improving sector performance series.", "improving power performance series.", "improving food performance series.", "improving peer 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 voluntary performance evidence.") or ("improving poor performance series."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving voluntary performance evidence.") OR ("improving poor performance series.")).
• Removing quotes may allow a broader search: (( improving voluntary performance evidence. ) or ( improving poor performance series. )).
• Adding a wildcard symbol may retrieve word variants: (("improving voluntary performance evidence.") or ("improving poor performance series."))*.