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