Perhaps you should try some spelling variations:
"improving human performance series." » "improving human performance studies.", "improving human performance size.", "improving loan performance series.", "improving thermal performance series.", "improving chain performance series."
"Improving firm performance series." » "Improving firm performance studies.", "Improving firm performance size."
improving firm » improving firms, improving air, improving erm

You may be able to get more results by adjusting your search query.

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