Perhaps you should try some spelling variations:
"improve external performance series." » "improve emotional performance series.", "improve materials performance series.", "improve thermal performance series.", "improve external performance studies.", "improve external performance size.", "improve maternal performance series.", "improving external performance series.", "approval external performance series."
"improving national performance reports." » "improving regional performance reports.", "improving operational performance reports.", "improving educational performance reports.", "improving maternal performance reports.", "improving natural performance reports.", "improve national performance reports.", "improving national performance report."

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• Performing a fuzzy search may retrieve terms with similar spellings: (("improve external performance series.") or ("improving national performance reports."))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("improve external performance series.") OR ("improving national performance reports.")).
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