Perhaps you should try some spelling variations:
"improving brain performance series." » "improving brain performance studies.", "improving brain performance size.", "improving chain performance series.", "improving loan performance series.", "improving can performance series.", "improving rail performance series.", "improving air performance series.", "improving basic performance series."
"improving job performance series." » "improving job performance studies.", "improving job performance size.", "improving court performance series.", "improving co performance series.", "improving food performance series.", "improving crop 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 brain performance series.") or ("improving job performance series."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improving brain performance series.") OR ("improving job performance series.")).
• Removing quotes may allow a broader search: (( improving brain performance series. ) or ( improving job performance series. )).
• Adding a wildcard symbol may retrieve word variants: (("improving brain performance series.") or ("improving job performance series."))*.