Perhaps you should try some spelling variations:
"improving basic performance series." » "improving basic performance studies.", "improving basic performance size.", "improving based performance series.", "improving music performance series.", "improving baseline performance series.", "improving air performance series.", "improve basic performance series."
"improving fast performance evidence." » "improving past performance evidence.", "improving first performance evidence.", "improving test performance evidence.", "improving asset performance evidence.", "improving food performance evidence.", "improving air performance evidence."

Try widening your search to All Fields.

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