Perhaps you should try some spelling variations:
"improving contract performance measures." » "improving country performance measures.", "improving court performance measures.", "improving economic performance measures.", "improve contract performance measures.", "improving central performance measures.", "improve contrast performance measures."
"improving firm performance series." » "improving firm performance studies.", "improving firm performance size.", "improving firms performance series.", "improving air performance series.", "improving erm performance series."

Try widening your search to All Fields.

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