Perhaps you should try some spelling variations:
"improving preschool performance metrics." » "improving school performance metrics.", "improving prediction performance metrics.", "improving present performance metrics.", "improved preschools performance metrics.", "improving schools performance metrics.", "improving preschool performance measures."
"improving human performance series." » "improving human performance studies.", "improving human performance size.", "improving loan performance series.", "improving thermal performance series.", "improving chain performance series."

Try widening your search to All Fields.

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