Perhaps you should try some spelling variations:
"improving human performance series." » "improving human performance studies.", "improving human performance size.", "improving loan performance series.", "improving thermal performance series.", "improving chain performance series."
"improving plant performance evidence." » "improving port performance evidence.", "improving court performance evidence.", "improving land performance evidence.", "improving place performance evidence.", "improved plant performance evidence."

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