Perhaps you should try some spelling variations:
"improving voluntary performance measures." » "improving country performance measures.", "improving entry performance measures.", "improving budgetary performance measures."
"improving thermal performance metrics." » "improving thermal performance contracts.", "improving thermal performance contract.", "improving maternal performance metrics.", "improving rural performance metrics.", "improving human performance metrics."

Try widening your search to All Fields.

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